Bunny hopping
Chaining jumps and steering in mid-air so the character accelerates well past its normal top speed — a bug that became a skill.
Quake's movement code doesn't cap total speed, only the component of velocity in the direction the player wants to move. Strafe sideways while turning the mouse and you pick up fresh acceleration every frame; land and jump on the same instant and you dodge ground friction entirely. The result is a speed that keeps climbing for as long as the rhythm holds.
The code carried over into Half-Life and from there into Source, and competitive players built whole maps and game modes around it. Developers have since added speed penalties on landing, while speedrunners keep hunting for new variants. Few bugs have lived as long or had as many fans.