Hello,
I accidentally turned off a 40 amp double pole water heater breaker when I was trying to turn off a different circuit in an old Cutler-Hammer panel. I attempted to reset it 3 or 4 times; it would 'click' on and then trip instantly. I tried it again in 15 minutes and it did the same thing. Finally I tried over an hour later and it reset fine. My question is, is this likely a water heater load issue or just a bad breaker?

Sounds like a weak breaker to me. Was the heater drawing power when you did this? Did you try resetting with the heater turned off/down

I'm not sure, and no I did not try that. The heater is not easily accessible. If it happens again I'll change the breaker.

I would too, its not very expensive. Anytime I see a breaker acting funny I swap them out tobe on the safe side

The breaker should hold or reset with or without load. The CB is defective, not going to get better on its own, why wait, change it now.

Good time to confirm, since it is a 40 amp CB, it should have #8 wire for the branch circuit.