See what happens if two APM powered quads get too close to each other.
The innocent ArcticCoper was in loiter, the TBS Discovery was in stabilized trying to get close up. Slow motion footage shows how one motor of the AcrticCopter completely stops after hitting the TBS Discovery quad. The damaged propeller then drags one arm down of the TBS Discovery, you can even see the propeller bend.
Both quads survived, only damage was a clipped prop on the ArctiCopter.