This is more a case of my brother unwittingly pwning himself with my help, but I think it counts.
As a conservative Christian, I am abstaining from sex until marriage. My brother, a far- left atheist, regularly mocks me for my faith and claims that my abstinence means I'm a loser who'll be alone forever.
One night, the movie 'The 40 Year Old Virgin' was on TV. I'd never seen it before, but my brother had, and basically dragged me into the room where he was watching it, and laughingly said, "You NEED to watch this movie! You're just like him, you're gonna be a 40- year- old virgin!"
So, I watched it, it was alright, but not the comedy classic some people make it out to be. Apparently, for all my brother's mockery, he forgot how the movie ends.
SPOILERS!
Ultimately, the titular virgin played by Steve Carrell finds his true love, and loses his virginity to her on their wedding night! The movie that my brother thought he could use to make fun of my beliefs actually encouraged them! I brought that up, and my brother's gloating died down pretty freaking fast!