Here's an interesting bug/technique in HL2 called 'Accelerated Back Hopping' https://www.youtube.com/watch?v=NV-AWxqYAgc&t=41m43s

It's used in speedruns, but I believe is fixed these days. Apprently if you're bunnyhopping backwards, the game tries to limit the speed, but since you're going backwards the player ends up accelerating essentially indefinitely.