It happened to me couple of yrs ago, that I came back from NTUC with groceries, and while emptying out the bags at home I noticed a bell pepper that I didn't recollect purchasing. It wasn't mentioned on my bill too.
Turned out my then 2 year old had picked up the shiny green bell pepper for her amusement, and then popped it into the bag without me noticing somehow!
I raced back to NTUC, explained them the situation, paid $1+ for the damn pepper and returned.
Now I realize I was so close to being taken to the police station