This glitch allows you to open any door in the game early. To do this, simply walk up to a door, and start left clicking (punching). I usually do this on the top half of the door, but it works on the bottom half as well. This works with an open hand, a keycard, or a gameband. After several left clicks, you will see a puff of smoke and some slimes. When you see this, right click the half of the door that you punched and it will open. The door may try to push you back as you enter. To avoid this, try backing up and heading straight for the door from the direction it's facing.

Video tutorial: