The X ABI has been broken hundreds (probably thousands) of times.
I unfondly remember the days of the CU computer lab where some applications would not display properly from the HP to the X terminals but worked from the Sun. I recall it was xv the image viewer that was usually good for breaking this stuff.
I remember some odd problems with that second, higher performing proprietary(I think it was?) X server Red Hat included with the boxed version 5.2.