Infinite Health Door
Infinite Health Door is used to gain a huge amount of health points. It's a mapping error, so it's only found in the map c2a5f. This trick is very useful for the rest of the game maps in a speedrun, since you can do unlimited damage boosts and survive.
For some reason* in this map the designer left both of the doors (likely copypasted) with -1 damage value, this HURTS you with -1 damage (which is negative negative => positive) per tick, which is once per frame.
Since the HEV (armor) reduces the damage you take at 5 regular points => 2 HEV and 1 HP and uses rounding for values less than 5, 1 regular point lost with HEV becomes 1 HEV and 0 HP lost. Thus you must have zero HEV otherwise HEV will increase and health will remain the same. The value is a 32bit integer so that means a maximum of 2147483647hp. To reach that value, you'd have to stand in the health door for 24 days (576 hours).
Raising the FPS in this helps a lot as you get n amount of hp per second related to your FPS. so 20fps is 20hp/s, 1000fps is 1000hp/s and so on.. The main trick here is to get the door to hurt you, easiest way is to get stuck inside it as if it's trying to kill you.
- [the leading theory is that Barney on this map, perhaps more so with older AI, would get stuck in these two doors frequently enough and die (ending the game) that the hurt value was hastily modified as a quick fix, (0 might have been a better number to choose) perhaps just for play-testing/debugging purposes but was forgot to be switched back or just left due to hast of making a deadline and not wanting Barney to be dying on players and ending the game.]
Simply make the guard or grunt to open the door (see c2a5f), and while it's opening - try to get stuck into it, or have it crush you. You'll gain health as long as you stay stuck in the door.