There are others who contribute to this site that will be able to answer much better than I. But not with the info you've given.
For starters no one here knows anything about you. Are you a GM of an SW store? Or an employee at one? Or just a painter with an old can of X and sold it on craigslist or something? Or just an amateur/homeowner who maybe got stuck with an unused purchase? Or you go to storage unit auctions and got this can of paint from that? Or what? We don't know. Go here and tell people who you are and what your painting industry role & background are:
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Then you'll need to say more about the sale. All we've got it that you "mixed it up good." ??? Have no idea what that means. Did you need to tint it and then it went on a mechanical shaker? (In this case the lid would have been off and you'd be able to report on the condition of the contents when you tinted/sold it). Or did you just pick it up and do the old hula shake? Or what?
I've spent some time on car repair help websites too. What you posted is the equivalent of "my car is making a funny growling noise. What it is?"