Well the possible but not likely happened. I have small thermometers hanging into my pool. They're about 3/4" diameter tube shaped types. Turns out, when my TigerShark climbs the wall, they will eventually get stuck in just the wrong place: between the brush roller and the body of the cleaner.
My cleaner hung there for likely hours, helplessly churning on the thermometer, wreaking havoc on the roller. I was too mortified to think about snapping a useful photo... Several ball bearings on the bottom of the pool confirmed my fears. One of the roller bearings was broken, leaving the contents on the pool floor.
Took the robot apart and here's the aftermath:
Hopefully a replacement bearing will be all it needs. Let this be a lesson. Lift those thermometers out when starting your robots.