Not then giving a placebo to some guys and making them think they're unlikely to catch HIV though, probably resulting in a few catching it. That has my luck written all over it.
I want to know how the experiment worked though, how'd they know the ones who didn't get HIV at the end of the 2 years even had sex with someone who had HIV during that period? Maybe all the guys who caught it had sex with a HIV positive guy, and the other people didn't have sex with one...

Bit dangerous if they encouraged these people to 'get it on' with a HIV positive person.