I don't see what's off-topic about this. Seems a perfectly reasonable beginner question, doesn't it?
Basically, if you change and re-build part of an application, and not other parts that would be affected, you can mess it up in a variety of ways, including this one. That's a mis-build and not really terribly interesting beyond that, I think.
In situations where you want to prevent modified code being put into your application, Java has various protection mechanisms available, like SecurityManager, Jar signing, and all sorts of things that I'm afraid I never use, so can't advise further on.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
Discussed to death here. Can you please let us know who is putting this bee in your bonnet, so we can look out for it in the future? Both people who posted this question posted it multiple times in multiple forums, so I'm thinking it's a job interview question or something.