Once ate a Vanilla Gu about halfway through a trail running ultramarathon that just didn't taste like vanilla, more like butterscotch or something. Looked at the date and saw that it was about 4 years expired. Stomach was pretty dodgy to begin with, that just threw it over the edge. It all came back up. No harm done other than all the water that came with it. (this race had no aid stations so you carried everything, didn't like seeing about a quart of water go to waste like that)