Is there a limit on how hot you can superheat water at 1 atm in a perfect container? What about in a microwave?
Amazingly someone has written a paper on this very subject (some people have far too much free time) and you can find it here. I wish I could claim this was my encyclopaedic knowledge of Physics at work, but it was just some lucky Googling.
Anyhow, the theoretical limit for superheating of water is (astonishingly) about 600K, but in real life you wouldn't get anywhere near that.