When Rigidbody bumps into a CharacterController, callbacks to OnCollisionEnter arrive a couple frames too late.

To reproduce:

1. Download attached project "Unity2017Repros.zip" and open in Unity
2. Open "LateCCOnCollisionEnter" scene
3. Enter Play mode
4. After few seconds exit Play mode and inspect Console:

- Starting at t = 0.6, the CharacterController (CC) is told to Move(0.1, 0, 0) each frame, making a bee-line for the cube.
- At t = 0.78, the CC bumps into the cube and only moves 0.03 in x rather than the full 0.1 as requested by the Move(). The capsule is now “stuck” at x = 1.12. This is all as expected, as 1.12 + 0.08 + 0.5 + 1.0 = 2.7. (1.12 is the CC’s center, 0.08 is the CC’s Skin Width, 0.5 is the CC’s radius, 1.0 is the distance from the cube’s center to the face the CC hit, and 2.7 is the cube’s x position.)
- Move() continues to be called on the CC every frame, but it doesn’t get anywhere for now.
- At t = 1.2, the cube starts to move slowly towards the CC, by (-0.03, 0, 0) each frame. For a few frames, this simply reduces the separation between the cube and the CC, “eating into” the CC’s Skin Width “buffer”.
- At t = 1.24, the cube moves to x = 2.61, and so now the cube and the CC are truly overlapping. We receive no OnCollisionEnter during that physics step, however. (The cube is listening for this callback.)
- At t = 1.26, the capsule (which always tries to move first), now that it is penetrating the cube, freely moves from 1.12 to 1.22. No hits reported by OnControllerColliderHit, so indeed the CC sees itself as penetrating the cube. The cube then moves an additional -0.03 in x. During the next physics step, we still do not see any calls to OnColliderEnter however.
- At t = 1.28, the capsule continues to move unimpeded from 1.22 to 1.32. The cube moves another -0.03 in x. Then, finally, during the next physics step the cube receives the OnColliderEnter callback that it has collided with the CC. The cube, during the callback, tries to “undo” its movement from the last frame by moving +0.03 in x. (And from this point on it no longer tries to move -0.03 in x each frame.) During the callback, we also see that the cube has already moved in the +x direction, presumably as a collision response to its overlap with the CC. It’s not enough to separate them though, and from then on the CC continues to take steps forward, and the cube continues to get “pushed along” by the CC (even though CC.Move() isn’t generally supposed to be able to push things).

Notes:
- It really seems there’s a delay of 2 frames in reporting OnCollisionEnter and not a specific distance threshold. If, for example, the cube only moves -0.003 in x each frame, it takes a lot longer for the cube to traverse the CC’s Skin Width, but once it does, the callback still occurs exactly 2 frames late.
- If you add a Capsule Collider (with the same dimensions as the CC) to the capsule gameObject, collisions are then reported correctly.

Reproduced on Unity 2017.1.3p1, 2017.2.1p3, 2017.3.0p4, 2018.1.0b5 and 2018.2.0a1