I'm not a giant harry potter fan, but I went to the release party in Baltimore, MD where there were over 3000 people there (yea, it was insane). I was there with some friends.
Now, as I stated I'm not a Harry Potter fan, I already knew the ending, and if anyone did this at the store I was at, I was prepared to give them a few medical bills for their stupidity. It's not a good prank, hell, it's not even a prank, it's called being an asshole, and all of them should just:

untill dead. If you don't care about Harry Potter, congradufuckinglations. There are people who do, why do you have to go out of your way, to stroke your ego to ruin other peoples fun?
It's not funny, get over yourselves.