Beer that sits around for a while and/or is subjected to temperature shock will form a flavor, probably (this is a guess) due to the formation of weird carbohydrate acetals, and maybe a few ethers. Maybe it's residual sulfur or somesuch. Whatever this smell & taste is, it seems to be particularly bad in Budweiser.

This flavor is not at all pleasant, and it will take a few before you can't taste it anymore.