Alright, here's a really evil one i did:
We had a guy in our class who was completely controlled by his feelings and he was plain easy to manipulate. He was in love with a girl in the class (who in my opinion is an ugly [bad word] but everyone seems to think she's pretty). So we made a fake MSN of that girl, and we asked her to give the victim her "new" MSN adress. On a side note here, almost everyone in-and outside of the class knew about this prank BUT the victim. I find it hard to be so unlucky or dumb not to hear it because me and my friends were really nervous about people always talking about it too loud and they were talking about our plan even near the victim. And yet he didn't suspect anything because he was blinded by love. pfft...
Another side note here: We also had a fake adress of her "boyfriend" that was a character we made up(in reality the girl had a different boyfriend like... every week).
So, we sign in to the fake MSN, and start talking to him. We said some sweet words to him but nothing really suggestive, just nice, kind and generous stuff. Then he tought they were both in love or something because he said:
"My bro said he won't make me go away from the comp because he says he won't stand in between us."
So long story short, any lightly kind or encouraging(if even just barely remotely) thing he hears about this subject he immediately thinks they fit together and whatnot. In reality he didn't have a chance.
We also asked him to talk to her "boyfriend"(the character i mentioned above), and we started a nice MSN internet troll fight. Hell of a lulz!
And after that the girl asked to send the chat log of the whole conversation and since she had a similar intelligence as the victim she assumed from the reactions that we were saying suggestive things, rather than the things we said themselves, and so she told the victim the truth. Funny thing was, he was all angry on us, but forgave us, he said something like "that wasn't very nice of you", however he didn't have ONE bad word on the girl even tough he knew very well that she was neck deep in it just like the rest of us.
So now you don't only use other people's overly strong feelings for personal gain, but also for the lulz.