Exclamation mark

From ZDoom Wiki

The Exclamation Mark is displayed when there is an error in the map which involves actors. The possibility for this could be a missing sprite frames in the wad for the actor or an incorrect/unused DoomEdNum in the map. An error is usually printed to the console in this case.

Skulltag also uses this icon for a "medal" for getting killed 5 times without fragging anyone else with the phrase "You Fail it", which is now a common phrase used in the Doom community.

Randy drew this icon with Microsoft Paint.