Mine was a purple lipstick from Boots (for a Hallowe'en costume) when I was a student.
I'd bought quite a few bits there, and thought the total seemed cheap, so I queried it. The cashier said it was because there were offers on some on the things I'd bought, so I didn't think too much about it, went off to other shops etc., sorted the rest of my outfit.
It wasn't I finally got around to going through my receipts about six months later (
) that I realised the lipstick wasn't on there. By that point the branch of Boots had actually closed and moved so I didn't do anything about it. It has stuck in my head though!