I've gotten 2 mimes and 4 mazes... I was training smithing one day and had my inventory full of iron bars, and I got 2 mazes within the hour that day. There is a theory like this on the P2p board like this too and I think it has a point. Out of all the random events, MOM is definitly the most common, where he gives you the box that you can't drop and multiplies. So maybe if you have your inventory full, when he tries to give you the box, he can't because there's no place to put it, so instead he gives you a maze... Just a theory...