Ok this is probably the funnest glitch and its mostly used on rampart rumble.

1. Select your grenades
2. Go to the small waterfall at the seals spawn(or the mercs im not sure)
3. Lay down before the water.
4. Hold R + X and your grenade aimer thing should be red.
5. Move forward continuesly holdying R + X and you can crawl under/over water and up steps.


P.S. there is a glitch on iron coffin that this can be used for the get inside 1 of those control pads under ground.