Debug draw
An API for drawing throwaway lines, boxes and text straight into the game world, making invisible logic visible.
A raycast that misses, an AABB that is too big or a navmesh with a hole in it cannot be seen in the finished frame. The fix is a call like DrawDebugLine in Unreal or Debug.DrawLine in Unity: the code asks for a line from A to B in some colour, and the engine draws it over the scene for one frame or a few seconds. Physics engines such as Box2D and Bullet ship their own interfaces for drawing collision shapes the same way.
The API is nearly always immediate mode — nothing to create, nothing to clean up — because it has to be sprinkled into the middle of physics code without ceremony. It all compiles out of release builds, at least in theory.