I can reproduce this bug exactly using the nvidia driver & Xgl/compiz. Here is the backtrace:

jeff@dapper-desktop:~$ gdb `which xchat-gnome`
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This GDB was configured as "i486-linux-gnu"...(no debugging symbols found)
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".

(gdb) set args --sync
(gdb) run
Starting program: /usr/bin/xchat-gnome --sync
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---Type <return> to continue, or q <return> to quit---
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[Thread debugging using libthread_db enabled]
[New Thread 1995011776 (LWP 24360)]
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---Type <return> to continue, or q <return> to quit---
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---Type <return> to continue, or q <return> to quit---
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The program 'xchat-gnome' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadImplementation (server does not implement operation)'.
  (Details: serial 799 error_code 17 request_code 129 minor_code 5)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

Program exited with code 01.
(gdb)