The space marine makes many faces in the game depending on the player's status. One of these faces is a "shocked" face known colloquially amongst Doom fans as the "Ouch Face", which rarely makes an appearance due to a coding error in which the programmers used "(plyr->health - st_oldhealth > ST_MUCHPAIN)" instead of the correct expression "(st_oldhealth - plyr->health > ST_MUCHPAIN)". While this face was meant to show up if the player took 20 or more hit points worth of damage in a single tic, the error meant that it only displayed when the player "gained" 20 hit points while taking damage. An example of this can be seen when attempting to use the IDDQD cheat during the very end of E1M8: Phobos Anomaly.

A number of source ports fix this behaviour, allowing the Ouch face to display as originally intended.