For all that say pranks are harmless tell that to the family of this nurse.
You never know what can cause some one to reach the breaking point. Your "harmless" prank may be it. Suck to have to live with knowing your minute of laughter would be last thing in that persons mind.
Sigh... Yes, that was a mean one, but most pranks are harmless and fun at work. If you don't want to play, then don't, but take your soap box elsewhere.