If you load a Prolog file containing XPCE classes using plain Prolog, you sometimes get and sometimes don't get this error. What is the problem?

The :- pce_begin_class directive is handled using term_expansion/2 if XPCE is loaded. Unlike predicates, term_expansion rules are not autoloaded. Thus, if you reference an XPCE predicate first or otherwise force XPCE to load, all is fine, but if the first XPCE reference is a new class it fails.

What to do?

Load XPCE explicitly by starting a file or module that uses it with the load-command:

:- use_module(library(pce)).

:- pce_begin_class(my_window, window).

...

:- pce_end_class.