this didn't happen to me and I wasn't in on it, ... but it's probably the best prank I've heard of.

An 'acquaintance' of mine called the county health dept to ask if they could schedule an -- aids test for one of his buddies.  

He told the nurses at the health dept that his friend had been feeling a little 'off' for some time, and was unsure of just what was wrong with him, but was 'worried' that some of his past indicretions with drugs and sex might be catching up with him.  Unfortunately, he was too embarrassed to schedule the test himself. 

He asked them to schedule the test, and due to the 'shyness' of his buddy, insist that he come in aand submit the required sample for testing -- don't take 'no' for an answer.  And then he gave them the buddies cell phone number.

There were several around who witnessed the buddy receiving the phone call that insisted he come in to submit the needed sample.  First puzzlement as to why the health dept was calling, then embarassment that they thought he needed to take an aids test, then ... well you get the picture.

This same pair have been the subject of some cell phone pranks as well.  One such prank was to text message something embarassing (totally fabricated) about him to everyone on his contact list, another was to pick out someone on his contact list and just call them repeatedly from his phone, and hang up -- continuing until sure that is has innocent party infuriated.

Borderline cruel, but funny as all getout if you're just a bystander.