Most recently this happened with a snowboarding helmet, which was too small. Surely I'm not the only person who incorrectly guessed his snowboard-helmet-size (I've since learned it varies by brand).
In addition to helping the buyer, it would also help subsequent buyers, who would then know if an item is true to size or not.
* bought by mistake * no longer needed
Both of these are true if the item is the wrong size (unless you ordered the right size and they sent you the wrong one in which case "wrong item was sent" is appropriate).
I just pick random answers usually.