Condensation, from changes in temperature.
Small amounts of moisture from humidity.
The worst i have seen are watches with plastic crystals that have turned yellow.
I believe the were manufactured with bromine. Anyway, the plastic releases a gas when it deteriorates. And the gas can eat right through the hands I have seen some where the hands are just rusty flakes. Movements are generally ok, but the hands will be shot.
I think I have only seen this plastic crystal on a Rolex once, a small 28mm watch from the WWII era. I have seen it on several other brands though.
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