A few years back I started seeing the blue and red painted steel flanges on ABS and PVC closet flanges display corrosion. One toilet in particular, the flange itself was simply a round circle of rusty looking dust and the toilet would flop around like a fish in the bottom of a boat when someone would use it. The only thing holding the toilet in place was the supply tube and the ABS pipe portion of the flange.
I switched to using stainless steel flanges long ago. (I've seen too many distended and malformed PVC and ABS flanges to consider using all plastic flanges.)
I'm curious if anyone else has come across this.