Hearsay alert. The following has not been confirmed.

Sounds like it might have been diet soda. If it gets too hot and/or too old, the aspartame breaks down into its components: two amino acids and a methyl alcohol molecule. When this happens, I doubt that there's enough methanol to do you serious harm, but it sure doesn't taste sweet any more.

Fred

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"For shame, gentlemen, pack your evidence a little better against another time."
-- John Dryden, "The Vindication of The Duke of Guise" 1684