Turn a freewheel or sprocket hub into a fixed gear. It's called a suicide hub because the gear is threaded on, with another lock nut following it. This can come off if it's not reinforced extremely well. The biggest danger is for brakeless fixies relying on the fixed gear to stop.
This guide uses loctite, and a comment says you can weld it too.
I have no experience on it, just learned of it yesterday. Proceed with caution.