In high school in my cooking class, we were making fortune cookies, and we put our own messages into them. One of them had the "If your hand is bigger than your face, then you cancer." message in it. So, a girl in my group got that cookie, and tried it out. Everyone in the group was cracking up when she fell for it. Then, I said "I can't believe she fell for it! That's the oldest joke in the book!". She slapped me, since I was in on it and didn't warn her about the prank. She was the only non-asshole person in the group, so I felt guilty after that. I thought she'd never forgive me.
My brother's pulled a few pranks on me. Like when him and his friend were brewing tea, I drank some with them. Then, a few months later, they told me those weren't tea leaves, it was marajuana. I knew they were lying, so I was faking some drama. I yelled stuff like "My life is ruined! How could you do this to me!", and faked that I would perform Seppuku.