Software is complex. Typically, to get the "garden path" (the most common, positive-success sequence of logic) to work involves dozens of things going right. read more
The original bug inadvertently"fixed" or bypassed a second bug (not directly related to the first bug). By fixing the original bug it exposed the 2nd bug because it no longer solved the secondary bug. You now have to find the secondary bug. Moral,"two wrongs sometimes do make a right". 2. read more