"Second Server Glitch"
There used to be a glitch where quickly running in and out of a door(or other warp) could cause the game to see your character inside the building while you're actually running outside on the gmap on your screen.
Some things wouldn't load outside in this glitched state allowing you to simply walk out of the boundaries of the map.

If you tried to enter any door or other warp you'd be teleported back inside the building you started the glitch at, although at a random coordinate inside that level.
People used to abuse this during Nexus and Revolution parties to get behind the bars a lot since those areas used to be inaccessible unless you got dragged by an admin.