Water in the tubes ...
Flying in cloud, IR(R) lesson, the pressure instruments all started misbehaving - sticking, jerking and so on.
I (rightly or wrongly) wasn't too worried, as the altimeter appeared to be telling the truth if you ignored the occasional hiccoughs, and I was prepared to carry on flying, treating this as a real but not desperately serious emergency, but the instructor, sounding slightly panicky, took control. And got us out of the clouds and landed us without incident.
I later deduced water in the tubes. I've never seen anything like it in VMC, of course - anyone who tells you that the instruments don't know you're in cloud is lying, just like the people who'd have you believe that the engine doesn't know it's over water (my only other instrument failure was in IMC).