I accidentally knocked out the ball bearing on the back side of one of my motors ... I suspect because I was careless in letting the magnets "snap" the bell back on. If you do that, and if the center shaft is not perfectly well aligned, it can push the ball bearing out.
The symptom in my case was that it became very tricky to get the motor bell back on such that it rotated freely.
And then, even once you get it back on, under load (out in the pond yesterday) it can come right back out, and cause bad things to happen (the motor uses much more current than it should, the magnets bang against the electro magnets, the bell/prop shake and don't really turn very well).
I noticed something was wrong because when I asked the ROV to go straight, it would curve very strongly left because my right motor was messed up.
Anyway, I unscrewed the motor mount, saw that the ball bearing had nearly fallen out in the back, pushed it back in, remounted the motor, and very slowly put the bell back on.
It could be I have a bad motor (whose ball bearing falls out too easily) ... has anyone else seen this?