diff --git a/lib/kwinglobals.cpp b/lib/kwinglobals.cpp
index 78a0887a9f..5c30fe4e48 100644
--- a/lib/kwinglobals.cpp
+++ b/lib/kwinglobals.cpp
@@ -23,6 +23,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <config-workspace.h>
 #include <config-X11.h>
 
+#include <assert.h>
+
 #include <kdebug.h>
 
 #include <X11/Xlib.h>
@@ -65,70 +67,101 @@ int Extensions::render_version = 0;
 bool Extensions::has_glx = false;
 bool Extensions::has_sync = false;
 int Extensions::sync_event_base = 0;
+// for fillExtensionsData()
+const char* Extensions::data_extensions[ 32 ];
+int Extensions::data_nextensions;
+int Extensions::data_opcodes[ 32 ];
+int Extensions::data_error_bases[ 32 ];
+
+void Extensions::addData( const char* name )
+    {
+    assert( data_nextensions < 32 );
+    int opcode, event_base, error_base;
+    XQueryExtension( display(), name, &opcode, &event_base, &error_base );
+    data_extensions[ data_nextensions ] = name;
+    data_opcodes[ data_nextensions ] = opcode;
+    data_error_bases[ data_nextensions ] = error_base;
+    ++data_nextensions;
+    }
 
 void Extensions::init()
     {
-    int dummy;
+    int event_base, error_base;
+    data_nextensions = 0;
     shape_version = 0;
-    if( XShapeQueryExtension( display(), &shape_event_base, &dummy ))
+    if( XShapeQueryExtension( display(), &shape_event_base, &error_base ))
         {
         int major, minor;
         if( XShapeQueryVersion( display(), &major, &minor ))
+            {
             shape_version = major * 0x10 + minor;
+            addData( "SHAPE" );
+            }
         }
 #ifdef HAVE_XRANDR
-    has_randr = XRRQueryExtension( display(), &randr_event_base, &dummy );
+    has_randr = XRRQueryExtension( display(), &randr_event_base, &error_base );
     if( has_randr )
         {
         int major, minor;
         XRRQueryVersion( display(), &major, &minor );
         has_randr = ( major > 1 || ( major == 1 && minor >= 1 ) );
+        addData( "RANDR" );
         }
 #else
     has_randr = false;
 #endif
 #ifdef HAVE_XDAMAGE
-    has_damage = XDamageQueryExtension( display(), &damage_event_base, &dummy );
+    has_damage = XDamageQueryExtension( display(), &damage_event_base, &error_base );
+    if( has_damage )
+        addData( "DAMAGE" );
 #else
     has_damage = false;
 #endif
     composite_version = 0;
 #ifdef HAVE_XCOMPOSITE
-    if( XCompositeQueryExtension( display(), &dummy, &dummy ))
+    if( XCompositeQueryExtension( display(), &event_base, &error_base ))
         {
         int major = 0, minor = 0;
         XCompositeQueryVersion( display(), &major, &minor );
         composite_version = major * 0x10 + minor;
+        addData( "Composite" );
         }
 #endif
     fixes_version = 0;
 #ifdef HAVE_XFIXES
-    if( XFixesQueryExtension( display(), &dummy, &dummy ))
+    if( XFixesQueryExtension( display(), &event_base, &error_base ))
         {
         int major = 0, minor = 0;
         XFixesQueryVersion( display(), &major, &minor );
         fixes_version = major * 0x10 + minor;
+        addData( "XFIXES" );
         }
 #endif
     render_version = 0;
 #ifdef HAVE_XRENDER
-    if( XRenderQueryExtension( display(), &dummy, &dummy ))
+    if( XRenderQueryExtension( display(), &event_base, &error_base ))
         {
         int major = 0, minor = 0;
         XRenderQueryVersion( display(), &major, &minor );
         render_version = major * 0x10 + minor;
+        addData( "RENDER" );
         }
 #endif
     has_glx = false;
 #ifdef HAVE_OPENGL
-    has_glx = glXQueryExtension( display(), &dummy, &dummy );
+    has_glx = glXQueryExtension( display(), &event_base, &error_base );
+    if( has_glx )
+        addData( "GLX" );
 #endif
 #ifdef HAVE_XSYNC
-    if( XSyncQueryExtension( display(), &sync_event_base, &dummy ))
+    if( XSyncQueryExtension( display(), &sync_event_base, &error_base ))
         {
         int major = 0, minor = 0;
         if( XSyncInitialize( display(), &major, &minor ))
+            {
             has_sync = true;
+            addData( "SYNC" );
+            }
         }
 #endif
     kDebug( 1212 ) << "Extensions: shape: 0x" << QString::number( shape_version, 16 )
@@ -137,6 +170,14 @@ void Extensions::init()
         << " fixes: 0x" << QString::number( fixes_version, 16 ) << endl;
     }
 
+void Extensions::fillExtensionsData( const char**& extensions, int& nextensions, int*&opcodes, int*& error_bases )
+    {
+    extensions = data_extensions;
+    nextensions = data_nextensions;
+    opcodes = data_opcodes;
+    error_bases = data_error_bases;
+    }
+
 int Extensions::shapeNotifyEvent()
     {
     return shape_event_base + ShapeNotify;
diff --git a/lib/kwinglobals.h b/lib/kwinglobals.h
index 3990ccbf33..cf4923db42 100644
--- a/lib/kwinglobals.h
+++ b/lib/kwinglobals.h
@@ -128,7 +128,9 @@ class KWIN_EXPORT Extensions
         static bool glxAvailable() { return has_glx; }
         static bool syncAvailable() { return has_sync; }
         static int syncAlarmNotifyEvent();
+        static void fillExtensionsData( const char**& extensions, int& nextensions, int*&majors, int*& error_bases );
     private:
+        static void addData( const char* name );
         static int shape_version;
         static int shape_event_base;
         static bool has_randr;
@@ -141,6 +143,10 @@ class KWIN_EXPORT Extensions
         static bool has_glx;
         static bool has_sync;
         static int sync_event_base;
+        static const char* data_extensions[ 32 ];
+        static int data_nextensions;
+        static int data_opcodes[ 32 ];
+        static int data_error_bases[ 32 ];
     };
 
 } // namespace
diff --git a/main.cpp b/main.cpp
index 4e308a94e6..07f768fbfa 100644
--- a/main.cpp
+++ b/main.cpp
@@ -36,6 +36,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <QX11Info>
 #include <stdio.h>
 #include <fixx11h.h>
+#include <kxerrorhandler.h>
 #include <QtDBus/QtDBus>
 
 #include <kdialog.h>
@@ -72,10 +73,93 @@ static bool initting = false;
 // and -rdynamic in CXXFLAGS for kBacktrace() to work.
 static bool kwin_sync = false;
 
+// this is copied from KXErrorHandler and modified to explicitly use known extensions
+static QByteArray errorMessage( const XErrorEvent& event, Display* dpy )
+    { // "Error: <error> (<value>), Request: <request>(<value>), Resource: <value>"
+    QByteArray ret;
+    char tmp[ 256 ];
+    char num[ 256 ];
+    if( event.request_code < 128 ) // core request
+        {
+        XGetErrorText( dpy, event.error_code, tmp, 255 );
+        if( char* paren = strchr( tmp, '(' )) // the explanation in parentheses just makes
+            *paren = '\0';                     // it more verbose and is not really useful
+        // the various casts are to get overloads non-ambiguous :-/
+        ret = QByteArray( "error: " ) + (const char*)tmp + '[' + QByteArray::number( event.error_code ) + ']';
+        sprintf( num, "%d", event.request_code );
+        XGetErrorDatabaseText( dpy, "XRequest", num, "<unknown>", tmp, 256 );
+        ret += QByteArray( ", request: " ) + (const char*)tmp + '[' + QByteArray::number( event.request_code ) + ']';
+        if( event.resourceid != 0 )
+            ret += QByteArray( ", resource: 0x" ) + QByteArray::number( (qlonglong)event.resourceid, 16 );
+        }
+    else // extensions
+        {
+        // XGetErrorText() currently has a bug that makes it fail to find text
+        // for some errors (when error==error_base), also XGetErrorDatabaseText()
+        // requires the right extension name, so it is needed to get info about
+        // all extensions. However that is almost impossible:
+        // - Xlib itself has it, but in internal data.
+        // - Opening another X connection now can cause deadlock with server grabs.
+        // - Fetching it at startup means a bunch of roundtrips.
+
+        // KWin here explicitly uses known extensions.        
+        int nextensions;
+        const char** extensions;
+        int* majors;
+        int* error_bases;
+        Extensions::fillExtensionsData( extensions, nextensions, majors, error_bases );
+        XGetErrorText( dpy, event.error_code, tmp, 255 );
+        int index = -1;
+        int base = 0;
+        for( int i = 0;
+             i < nextensions;
+             ++i )
+            if( error_bases[ i ] != 0
+                && event.error_code >= error_bases[ i ] && ( index == -1 || error_bases[ i ] > base ))
+                {
+                index = i;
+                base = error_bases[ i ];
+                }
+        if( tmp == QString::number( event.error_code )) // XGetErrorText() failed,
+            { // or it has a bug that causes not finding all errors, check ourselves
+            if( index != -1 )
+                {
+                snprintf( num, 255, "%s.%d", extensions[ index ], event.error_code - base );
+                XGetErrorDatabaseText( dpy, "XProtoError", num, "<unknown>", tmp, 255 );
+                }
+            else
+                strcpy( tmp, "<unknown>" );
+            }
+        if( char* paren = strchr( tmp, '(' ))
+            *paren = '\0';
+        if( index != -1 )
+            ret = QByteArray( "error: " ) + (const char*)tmp + '[' + (const char*)extensions[ index ]
+                + '+' + QByteArray::number( event.error_code - base ) + ']';
+        else
+            ret = QByteArray( "error: " ) + (const char*)tmp + '[' + QByteArray::number( event.error_code ) + ']';
+        tmp[ 0 ] = '\0';
+        for( int i = 0;
+             i < nextensions;
+             ++i )
+            if( majors[ i ] == event.request_code )
+                {
+                snprintf( num, 255, "%s.%d", extensions[ i ], event.minor_code );
+                XGetErrorDatabaseText( dpy, "XRequest", num, "<unknown>", tmp, 255 );
+                ret += QByteArray( ", request: " ) + (const char*)tmp + '[' + (const char*)extensions[ i ] + '+'
+                    + QByteArray::number( event.minor_code ) + ']';
+                }
+        if( tmp[ 0 ] == '\0' ) // not found???
+            ret += QByteArray( ", request <unknown> [" ) + QByteArray::number( event.request_code ) + ':'
+                + QByteArray::number( event.minor_code ) + ']';
+        if( event.resourceid != 0 )
+            ret += QByteArray( ", resource: 0x" ) + QByteArray::number( (qlonglong)event.resourceid, 16 );
+        }
+    return ret;
+    }
+
 static
 int x11ErrorHandler(Display *d, XErrorEvent *e)
     {
-    char msg[80], req[80], number[80];
     bool ignore_badwindow = true; //maybe temporary
 
     if (initting &&
@@ -92,11 +176,8 @@ int x11ErrorHandler(Display *d, XErrorEvent *e)
     if (ignore_badwindow && (e->error_code == BadWindow || e->error_code == BadColor))
         return 0;
 
-    XGetErrorText(d, e->error_code, msg, sizeof(msg));
-    sprintf(number, "%d", e->request_code);
-    XGetErrorDatabaseText(d, "XRequest", number, "<unknown>", req, sizeof(req));
-
-    fprintf(stderr, "kwin: %s(0x%lx): %s\n", req, e->resourceid, msg);
+//    fprintf( stderr, "kwin: X Error (%s)\n", KXErrorHandler::errorMessage( *e, d ).data());
+    fprintf( stderr, "kwin: X Error (%s)\n", errorMessage( *e, d ).data());
 
     if( kwin_sync )
 	kDebug() << kBacktrace();