Every Call of Duty game has been rife with glitches which have either defined, or broken the multiplayer mode—at least until they were fixed. Modern Warfare 3 is no different.
That's not to say that it's a broken game or anything like that—only that there are a number of spots, like hidden areas and out-of-bounds locations in the maps that weren't previously found during the game's Q&A. Activision can certainly count on 9 million players to discover glitches even their best testers couldn't find.
First up is a video from RSPproductionz that showcases various tricks and glitches in the maps Outpost, Downturn, Resistance, Bootleg, and Arkaden.
RSPproductionz has also produced a video that shows you how to get out of the map in Fallen. This glitch allows you to stand outside the boundaries of the map, reachable only by strafejumping.
A similar trick can be done on the single player mission called Fatal Extraction.