Reproduction steps:
1. Open my attached project ("case851605_repro")
2. Open the "main" scene
3. Enter Play mode
4. Notice how when the Sphere collides with the Cube the console throws collision Debugs
5. Open either the "cube.cs" or the "sphere.cs"
6. Comment out the "OnCollisionEnter" method in one of them

Result: the OnCollisionEnter method is not called, even though the collision happened

This only happens if one of the objects has a CharacterController component on it.

Won't Fix: Character Controller is being deprecated.

Reproducible on: 5.3.7p1, 5.4.3p3, 5.5.0f3, 5.6.0b1