I had to look it up ichi.
Just in case not everyone in the world who wants to, has seen/read Harry Potter yet, I've spoilered this:
Ron (the brother), had a pet rat. Very late in the series it turned out that the rat was really someone called Peter Pettigrew who had been transformed years before. The twins had the Marauder's Map at one stage. It showed where everyone at Hogwarts was, so the word Peter should have shown up wherever the rat was. However, it is possible that the twins didn't see this as they had stopped using the map by the time Ron arrived at Hogwarts, or that they thought it was a mistake, which is what Harry thought.