A spare rear shock that my mates slipped in my bag as a joke, to see if I’d notice (it was my shock, a rather heavy 222mm long 5th Element with a steel spring on it). I did wonder why my bag felt quite heavy, but never opened it once inside of a weeks riding (in the Alps, I didn’t puncture), only found the shock on the last evening there when I was packing everything up to go home!
Have many many times ended up with out of date food, left lingering in the bottom of my bag. Which if you knew me, you’d find very odd, as I eat food like the world’s going to run out tomorrow!