I didn't really have time to stop and call anyone so I just let it be, but earlier today I encountered a psychopath in an 18-wheeler. He was coming out of a weigh station behind another truck - who was already up to the speed limit - when he decided to cross over the solid white lines to pass on the left while still on the one-lane on-ramp. The other truck was forced onto the shoulder, at which point our subject decided to back off. Once he was properly behind the other truck again, though, he got within five feet of it, going around 70 mph, (and that's no exaggeration. I slowed down significantly expecting a crash.) and just hung out around there until I made my way around, at which point he merged into my lane without signalling and followed me closely enough that I could only see his grille in my mirror. He stayed behind me until we came across an accident scene with police, and then he backed off and took the very next exit. Neither I nor the other trucker did anything to upset him, but it seems like his only goal was to cause both of us to crash.
Also there was a truck towing a car with the magnetic taillights switched. That way when he signaled right, the tow car signaled left. I tried to catch up to tell him, but he was too fast. Hopefully didn't cause any problems.