That or a short. My first iMac G3 (see other thread) did that, then one night went berserk, emitted a massive spark-crack and wouldn't power up again after that. (It also blew out the CX5M on the same circuit, which really p*ssed me off.) This new one doesn't do it, but I imagine it's a matter of time. I'm thinking of putting it on its own surge protector to protect the other machines from *it*.
Anyway, I agree with mcdermd; it's a blueberry slot loader.