I was flying back from Philly to KC (Lawrence) once, with a connection through O'Hare, except I didn't need to change planes because it was the same flight number, so I got to stay on the plane. We sat there for 5 hours because they needed an engineer somewhere (Dallas, I assume, because it was AA) to sign off on a repair. The repair? Changing the placard (sticker, really) on a lavatory ...
Ahhh, the joys of traveling.