You'll typically get these errors if you are writing a multi-threaded
application and are making X calls from more than one thread -- one of the
more common new ways to introduce memory corruption into Xlib (using bogus
pointers is another, as is mixing up XFree/XtFree/free calls. Even an
operation as simple as XSendEvent can't be called from a second thread.).
Prior to X11R6, X doesn't support multi-threading; check the X11R6
documentation for how to write a threaded application safely with X11R6 and
later versions of X (including being sure to enable Xlib's multi-thread
support).

Common widget sets are also unsafe to use except from within one thread.  The
versions of Motif from OSF, for example, offer no support for
multi-threading.

Some parts © 2009 Advameg, Inc.