[Yosemite Sam]

I may be way off base here but many brands "Oatey" included, contain Acetone. I believe the Acetone evaporates, causing the cement to turn to jelly. Freezing could also play a part in the jelling process, but my bucket of plumbing tools normally lives in the basement where it doesn't freeze and many brands will turn to jelly anyway.

I have started using "Black Swan" brand "3 way cement" (there is no Acetone listed on the label). The can that I have here is probably 2 years old and gets used occasionally, it still looks good today.

I have considered adding a little Acetone to cans of other brands that have jelled, just to see if it would come back to normal, but who has time for such nonsense?

But yes you're right, open a can of Oatey, finish the job and throw the rest away...