This is what happened on yesterday's AA 46 ORD-LHR: after boarding, which was already delayed by ca. 30 mins. (due to servicing/maintenance issues?) the flight captain announced that they were ready to go but were missing the log book required on int'l flights (joking that it must be the mechanics who presumable were holding it hostage). After about an hour it was announced that the log book had been delivered, and aircraft pushed back from the terminal.
Some brief moments later the cabin lights suddenly failed (except for battery powered emergency lighting?), and the captain announced a failure of the auxiliary power unit. They pushed back to the gate to have the aircraft inspected, to conclude (after maybe another hour) that it was "probably due to a glitch" and that the auxiliary power unit was not really needed for flying because it was only used to start the engines, and they would start one engine while still connected to the gate's power and later on start the second one from the battery (at least this is what they said). This having been said, the captain added that again the log book was missing. After some more minutes, apparently that issue could be resolved again, and the flight left for LHR with a ca. 3 hours delay, of which it was able to make up about 30 minutes on the way.
The aircraft was an aging B777-200.
I wonder whether this is a common issue with AA and/or that particular aircraft type. Maybe someone with more flight experience with them or other insight might want to comment.