XSetBackgroundPixmap question

[James Buren]

> The combination of the documentation and the observed behaviour
> implies that the X server *may* copy the pixmap data or it may just
> keep a reference. Either way, deleting the pixmap after setting it
> should be fine, but modifying a pixmap which has been used as a
> background pixmap has undefined behaviour.

What is undefined behavior? Modifying a pixmap that is still used
as a background pixmap, modifying a pixmap that was used as
a background pixmap but has since been replaced, or both of these
cases?