In chapter one, right near the beginning where you throw a propane tank at some people that takes out the bridge, and right after you shoot the larger propane tank across the gap, get across via the big crane-like object that falls across as you normally would. Don't go near the tractor you have to hotwire or you won't be able to do the glitch. Right after you make it across, look to the right of the screen right beside you. There are two small wooden box-like objects (I'm assuming they are used for carrying things with a forklift). One of the box-like objects is leaning against the other which makes something like a ramp. Walk up the box-like object while pushing yourself against the short, small wall they are against. Keep walking and you will go onto the other box that is flat against the ground. Walk back and forth along the wall on these two box-like objects and eventually your feet will fall through one of them. Change the angle of your camera with the right analog to see. To get out of the object, jump.
Submitted by: Mitchell VanRassel