Fatal errors under X11 include I/O errors or system call errors, such as irrecoverable network problems, attempts to establish a connection to non-existent, or non-accessible server, etc.
Since X automatically exits after detecting a fatal error and gives no real chance of correcting the problem, ProXL uses the default X fatal error handler, which just prints a message and exists.
If you feel you need to change this handler, we suggest that you consult the X11 documentation.