"How, exactly, does a repair bot fail?" I hear you ask.
Well, besides the fact that it's damn expensive to repair using a bot (not being affiliated with any factions and thus not subject to any reputation discounts), here's how:
...it walked under the stairs.
Luckily, while the bot failed, the PuG didn't - 25man Sartharion was one-shotted with 2 drakes up... and we didn't even use Ventrilo/Teamspeak. How's that for quality PuGgage?!
(If you're wondering about the bot - people had forgotten to repair before coming)