Ha ha ha ha ha. This shirt makes me laugh a lot. I guess this is a public service message from the NMMA (National Magic Missile Association).
This reminds of the episode of Harvey Birdman where Quick Draw McGraw is arrested for concealed guitar violation and opts to fight in open court, openly supported by the Guitar Lobby. Hilarious, and Charlton Heston as the voice of Quick Draw, sheriff and guitar activists, is hilarious.
Back to the t-shirt. Here is a great video of a LARP guy casting magic missiles. I can't help but laugh, in spite of all my friends who LARP. Here is another clip from Reno 911 where another guy tries to explain how his friend got shot in the shoulder with an arrow. Also excellent.
I also found this site that claims to have a real life Book of Magic Spells. I wouldn't recommend actually giving them any money or trying any of these spells. In fact I would highly recommend that you don't have anything to do with them in any way, shape, or form except to laugh at their products. However, if you are the adventurous type with lots of disposable income go nuts and let me know how it goes. Before you do that, however, be sure to use some of that disposable income to buy a few t-shirts.
All prices are in USD.