My story won't win for "smallest animal", but it's a good one nonetheless.
Had an old pulp mill shut down with dozens and dozens of motor overload trip alarms. Whatever didn't trip, stopped because something around it had tripped. We figured there must have been a power surge or some other funny business and set about working through the various switchrooms pressing reset buttons. Once all the alarms cleared, we went up to the control room and gave the operators the all clear to restart. First motor that restarted tripped on overload. So did a dozen more. Hmmmmm...
It was about then that we noticed that roughly 1/3 of the lights in the building were out. Slowly, something in my brain went "ping".
We were missing a phase to the building. At this point I stopped, and wondered if this might be related to the other radio chatter I'd heard that morning from a different crew onsite, regarding a rodent that had met a rather unpleasant demise. It had crawled into a DB in an administration building, squeezed in between the insulated busbar section down the middle, and started munching. Eventually it cremated itself, but not before it had blown the fuse supplying the administration building, and also the upstream fuse supplying the whole switchroom that supplied both the administration building and the pulp mill. So much for circuit discrimination.
But the funniest thing about this was what the guys found when they removed the escutcheon. Have you ever seen the episode of Mr Bean where he decides to paint his loungeroom by wrapping everything in newspaper, sticking a firework in a tin of paint, and then leaving the room? Except, at the last second, someone comes in to retrieve their hat. The results are a perfectly white room, except for the silhouette of a man reaching for a hat on one wall.
Well, that's exactly what the back of this escutcheon looked like. Completely charred black, but with the perfect white silhouette of a rat, right down to the ears and tail. It was so ridiculous that it ended up mounted on the workshop wall as an art feature.