A few days ago, me and two colleagues tried to track down a very tricky bug. After some hours looking, we finally found it, and it was actually due to a misconception that I had about the workings of HashSet and HashMap. I'm not sure if I'm the only one that didn't know this, but it's very logical once you've found it out. You see, if you save an object in a HashSet, and then change the object in such a way that the hashCode changes, then you won't find that object in the Set anymore. It will still be there, you will still iterate over it, but if you ask for example set.contains(obj), then it will return false. If you iterate over the set, and call Iterator#remove, this will silently fail to remove anything, since the HashSet can't find the object you want to remove. So, if you save things in a HashSet or use them as keys in a HashMap, make sure that the object is immutable, otherwise you'll get extremely hard-to-find bugs.
Incidentally, one of the best newsletters about Java programming wrote about this issue ages ago. Regardless if you work with Java professionally or just for fun I implore you to subscribe to JavaSpecialists by Dr Heinz Max Kabutz. It can be found here.