Apart from brain failure
.... I would have to say blown lighbulbs.
You'd really have to go into specific aircraft here. A modern glass cockpit is obviously not going to have too many problems with things like noisy gyros.
On an airliner, most of the stuff in the logbooks relates to lighting, IFE, galley stuff... but where "real" stuff is concerned, I'd say pneumatics/aircon.
Rgds.. .Q.