last week I had an APU charger fried the backup battery... kinda important I guess.

I had one where the engines had an issue starting. Captain didnt go into detail. Had another where the AC regulator was broken, that was only an hour. Another one was a 747 was struck by lightning while on the ground and put it out of commission completely (they changed planes).

But my personal favorite was the manual release for the Captains chair. They had to climb into the wheel bay and change out the whole thing. Plane was 7 hours delayed for that one.