Load game with the command line: hl.exe -dev -toconsole. Then, press ~ if the orange text on the screen does not disappear. During gameplay, press ~ to display the text prompt and type in sv_cheats 1. Press ~ and then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
God mode. - /god
No clipping mode - /noclip
Opponents ignore character - /notarget
All weapons and ammo - impulse 101
Auto-reload enabled - +reload
Auto-reload disabled - -reload
Map select - /map
Third person view - /thirdperson
Adjust gravity - sv_gravity <-999 - 999999>
Obtain item - /give

Activate cheat mode, then activate the /map redE3M5 code to advance to episode 3, mission 5. Find the door that does not have a handle in that level. Enter the /noclip code and walk through the door to find another door covered with smiley faces.

Close the teleportation door on the lab technician. He will not open it back up. After teleporting to the mountains, look to your right to see the scientist floating. Watch him for a moment and he will walk in place in mid-air. Note: You can kill him. 

Shoot the security officer after putting the HEV suit on and collecting weapons from the storage room. He will say things such as "What do you want your tombstone to say?" or "I dont want to do this...".

When you shoot the weapons man or the science technician, the alarm will go off and auto-turrets will deploy from the roof and take you down.