Nintendo of Europe released a statement today acknowledging the existence of a game-freezing bug in the European release of Super Paper Mario. If you only play games for the excitement of seeing them fail, you're in luck. Everyone else: read where and how it happens, so you can avoid it.
The offending bug occurs in Chapter 2-2. A character named Mimi will speak to you and send you on your way. Before the next time you meet her, you should be able to find a key. If you speak to her again without first picking up the key, the game registers its disapproval with you. In other words, it freezes. This only occurs when playing the game in UK English, German, or Spanish.
Nintendo will have information soon about how to get a replacement copy, so you'll be able to talk to Mimi whenever you please. Until then, you know, don't do that.