I've had an AI and DI go tits in IMC. Vac pump went on strike and the AI toppled. Not a great problem really but I practise a lot. If I'm off somewhere with a mate in vmc and I'm flying a leg I'll get him to be safety lookout while I go head down for twenty minutes or so. I enjoy flying on instruments (I know, weird).
The worst scenario is to have an AI that isn't erecting as fast as it should and not noticing that the TC and AI are telling different stories. I've had that a few times, get level on the AI and wonder why you're slowly turning.