I think I have seen something like it before. I forgot wether it was mine or my friends, but it was more subtle, and was different. So, becuase they were both different I would believe that it was just a printing mistake. What may have happened is either a. it went under the printer twice but moved a little or b. they double print cards to make sure that they are visible, but it got moved and there for messed the card up. I'm not entirely sure about this, but I am pretty sure that they don't purposely double print cards like that.
Also, here is a related question. It too says that it is just a misprint, and that besides being misprinted, there is nothing special. Also, I dunno where you would use it, but it says you can't use it for any official tournaments. Oh and I guess you could say it's rare considering that there aren't many misprinted ones ;)