Because of asynchronicity, you may have executed many commands when you discover that a previous command has caused an error. A ProXL feature described in pxl-eh-eho-synchronize enables you to get around this problem during debugging.
There are two types of errors in X, fatal errors and recoverable errors. Fatal errors are handled by the server and terminate program execution. Recoverable errors are handled by the ProXL error handler, or a user-specified error handler.