An attempt to run "glxgears" on a Xserver which has a 30bit TrueColor visual
results in the following error (note that the Xserver also supports 24bit and
16bit TrueColor visuals):
-- snip --
% glxgears
X Error of failed request:  GLXBadContext
  Major opcode of failed request:  144 (GLX)
  Minor opcode of failed request:  5 (X_GLXMakeCurrent)
  Serial number of failed request:  39
  Current serial number in output stream:  39
-- snip --

A quick check added in glxgears shows that the requested visual depth and the
window depth are the same (30bit) so I assume this isn't the part which causes
problems.
On the Xserver side the following waring is printed:
-- snip --
Mesa warning: XMesaCreateWindowBuffer: depth mismatch between visual and window!
-- snip --

I've added the following debug code to the Mesa engine...
-- snip --
Index: xc/extras/Mesa/src/mesa/drivers/x11/xm_api.c
===================================================================
RCS file: /cvs/xorg/xc/extras/Mesa/src/mesa/drivers/x11/xm_api.c,v
retrieving revision 1.1.1.1
diff -u -2 -0 -r1.1.1.1 xm_api.c
--- xc/extras/Mesa/src/mesa/drivers/x11/xm_api.c        16 Jun 2004 09:19:01
-0000      1.1.1.1
+++ xc/extras/Mesa/src/mesa/drivers/x11/xm_api.c        31 Mar 2005 13:08:09
-0000
@@ -1734,48 +1734,53 @@
 #ifdef FX
    char *fxEnvVar;
 #endif
    int client = 0;
 
    XMesaBuffer b = alloc_xmesa_buffer();
    if (!b) {
       return NULL;
    }
 
    (void) c;
 
 #ifdef XFree86Server
    client = CLIENT_ID(((XMesaDrawable)w)->id);
 #endif
 
    assert(v);
 
 #ifdef XFree86Server
    if (GET_VISUAL_DEPTH(v) != ((XMesaDrawable)w)->depth) {
+      _mesa_warning(NULL, "XMesaCreateWindowBuffer: depth mismatch between
visual (%d) and window (%d)!\n",
+                    (int)GET_VISUAL_DEPTH(v), (int)((XMesaDrawable)w)->depth);
+      return NULL;
+   }
 #else
    XGetWindowAttributes( v->display, w, &attr );
 
    if (GET_VISUAL_DEPTH(v) != attr.depth) {
-#endif
-      _mesa_warning(NULL, "XMesaCreateWindowBuffer: depth mismatch between
visual and window!\n");
+      _mesa_warning(NULL, "XMesaCreateWindowBuffer: depth mismatch between
visual (%d) and window (%d)!\n",
+                    (int)GET_VISUAL_DEPTH(v), (int)attr.depth);
       return NULL;
    }
+#endif
-- snip --
... and then I get:
-- snip --
Mesa warning: XMesaCreateWindowBuffer: depth mismatch between visual (24) and
window (30)!
-- snip --

Any ideas what may be going wrong here ?