I've seen some weird interactions between materials too. A friend of mine had an old radio that was stored with the power cord wrapped around it and the rubber insulation reacted with the plastic housing somehow and they melted into each other. I had a roll of that thin yellow tape that you find covering the windings in a lot of ferrite transformers and a roll of a different kind of tape that was left sitting on it turned the edges of the yellow tape into rubbery goo in the places they were touching.

Worse than belts, sometimes rubber pinch rollers break down and turn into tar-like muck. That was a fairly common problem with 8-track tapes too, the rubber roller being built into the tape cartridge. I'd be curious to know just what leads to this, whether it's inevitable with some materials, an interaction with vapors that outgas from nearby materials, or environmental factors.