Because of historical reason AWT doesn't try to avoid XErrors, but uses XErrorHandler instead.
(perhaps it was done to improve our performance).
It is ok in most situation, but if AWT is used with some user's native code or in combination
with SWT. These errors may becaome visible for users and cause some "unpleasant" effects.
See for example:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=110163
https://bugs.eclipse.org/bugs/show_bug.cgi?id=142644

Thus it would be better to fix AWT so every XError shows a real problem. Not sure if it is
possible though :(