Crouch Save Glitch (JClip)
From SourceRuns
Crouch Save Glitch/Jared's clipping method (or JClip)
Jclip is an Old Engine technique for getting stuck in props in order to clip out-of-bounds. It is achieved by crouching, saving the game, loading the save, standing up and moving the object inside of the player box; the player can then repeatedly save and load while crouched to move out of ceilings or skyboxes. The discovery of this glitch make going out of bounds possible in any map with a ceiling.
Currently, this technique is only viable with Source Pause Tool, as saveloading is much different in OE than NE, and an Autohotkey script is also recommended.