I was out with some people once at karaoke and this guy got a phone call and left to talk to whomever was on the other line. He then came back in about ten minutes later and asked his wife to come out and talk to him. After they were gone about 5 minutes, they came back in and she was totally crying, telling us between sobs that his unit had just come up for deployment in Iraq, and he was leaving in 72 hours. On top of that, she had just gotten pregnant, and she couldn't believe it, etc... Everyone was SUPER sympathetic to this couple, telling them we really felt for them, we were sure he would be okay etc. etc.
And then after like 15-20 minutes of milking it they were like, haha, we were just kidding! And everyone was completely shellshocked, and then and only then did they realize that they had totally misjudged what an appropriate "punked!" was.
It wasn't even April Fools!