I quite like the surprise of listening to an album for the first time and suddenly hearing a track I wasn't expecting. An extra unlabeled track is good, but a hidden track that you come across by accident is even better. According to the excellent Wikipedia article there are quite a few different ways of hiding tracks (some more ingenious than others) and plenty of well known albums with extra tracks. What all hidden tracks have in common though is that they are not listed on the album cover.
So now we come back to Avril Lavigne's latest CD which I mentioned a few days ago. Take another look at the back of the album and you will notice that track 14, Alice, is labeled as a hidden track. That just has to be a mistake -- it can't be hidden if it's labeled! I'm guessing that they wanted to make it clear that it wasn't part of the main album (it's actually from the soundtrack to Tim Burton's recent film Alice in Wonderland), but why not just label it as a bonus track, or better still make it into a proper hidden track?