This isn't technically expired food but I'm taking this opportunity to share it because it's so damn retarded.
So, pretty late in the day I go downstairs to get something quick to eat. A quick scout of the kitchen and I spotted something on the side. It was a packet of butter (lurpak) and on top of it was a little brown thing in a packet. In my tired and hungry state I thought "oh hey that looks like a nice chocolate biscuit" completely ignorant of the fact that it was on a box of butter.
This was the thing, they come free with the butter for some reason: (SPOILER)
So yeah it was a seed pellet. I put the whole thing in my mouth and nearly vomited because it was smurfing gross.