Because the original mistyped new password had now been stored as the old password, the program thought I was mistyping the old password from then on. A real user probably would have given up in confusion after a few tries. Then , the next time she tried to log on, she wouldn't be able to. This is not an easy bug to work around. As a related example, here's a bug in prerelease version 0.7.6 of the AbiWord word processor.
Many programs manipulate internal data.
By looking at the screen, we're only really checking one of those things. Check all of the effects of an action. You might have no way to examine an internal data structure. If so, you need to check it indirectly, by seeing whether the further actions that first action is supposed to enable actually work. Well, a user might tinker further with the line spacing to get it just right.