Have you ever known your fish to fight?

Something I can't say I ever noticed when I had my fish is whether they ever fought or not. As much as I was doing a lot of work on the tank at the time when I had one, all my fish seemed to get on and I never had any worry about them fighting.

I do know though that sometimes fish can and will fight from time to time.

Have you ever known your fish to fight? Was it a recurring problem? How did you deal with it?