Also known as: guard-skipping, guard-jumping.
Most guards can be skipped by run-jumping through them at a very precise point:
Level 1 (example)
Falling through a specific point in the level causes the Prince to "wrap around" the map and appear at the screen above.
Floor and gate clipping
Various techniques can be used to make the Prince "clip through" a floor or a gate that is closed
Level 7 (again)
Fall damage crouch cancel
You can negate the "crouch wind-down" period that happens after Prince's two-story fall, by falling in a specific point (a corner between left wall and the floor).
Inactive chomper glitch
You can make a chomper inactive by falling two floors and grabbing the ledge.
Guard through-gate lure
You can make a guard advance towards you through a closed gate (exploiting a glitch that makes him think you're already on the other side of the gate for a moment).