I just had the flapper valve fail one of my home's toilets. The rubber had become spongy, sticky, and was beginning to disintegrate. It's so soft now that it rubs off with the slightest touch.
The valve was installed by the previous homeowner, so I don't know its history. However, since I've moved in, this toilet has been very seldom used. It's in the "guest" bathroom, and unfortunately have guests over now who are inconvenienced.
I'd like to understand what happened here, so I can avoid something like this happening again.