ULTRAKILL - Revolver Remodel Easter Egg
The revolver remodel in the WRATH update came with a small easter egg: a small pixel illustration of a dog hidden under the variant-specific information texture on the revolver's display. This doesn't seem like it was supposed to be visible without the use of third-party tools, but due to a bug, the Marksman's coin display is unaffected by the Vertex Warping setting, allowing us to see it in-game.
A similar effect can be achieved at extreme distances from the level origin where the game starts falling apart (this phenomenon has a name I'm pretty sure but unfortunately it eludes me). I'm fairly certain this is how folks at the New Blood Discord discovered it at first, though fortunately the Vertex Warping setting can be accessed at any set of level coordinates.