It's something like corrupt file, but it happens on certain memory adress, not with specific file.
In some circumstances some memory cell returning something else than was stored in it (eg. if ones are around, it return one (but there are much more possible errors)).
If it happen to be part of bitmap, you have one pixel wrong and you don't notice. But if something saying what program should do is there, then it will do something different than it should (which extremely probubly will cause crash).