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 for an actor's Spawn state, 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.
The Eternity Engine, although not derived from ZDoom, adopted it as well. Notably, it displays it (with some translucency added) in places where map authors have left a Doom Builder 3D preview camera.
Randy drew this icon with Microsoft Paint.