When reporting an error in the glXGetSelectedEvent path use X_GLXGetDrawableAttributes in the minorCode. This prevents confusing it with the glXSelectEvent path a bit.
--- AppleSGLX/trunk/glx_pbuffer.c 2009-02-22 01:12:49 UTC (rev 220)
+++ AppleSGLX/trunk/glx_pbuffer.c 2009-02-22 03:19:02 UTC (rev 221)
@@ -229,7 +229,7 @@
error.sequenceNumber = dpy->request;
error.type = X_Error;
error.majorCode = (gc) ? gc->majorOpcode : 0;
- error.minorCode = X_GLXChangeDrawableAttributes;
+ error.minorCode = X_GLXGetDrawableAttributes;
_XError(dpy, &error);
UnlockDisplay(dpy);