Here is an animation someone (Lusche?) put together. It shows the film OVERLAPPED of a single collision, but the views are from the two different computers.
As you can see, the plane that is flying strait and level are both planes overlapping. From one view the pony collides from behind, for the other computers view he didn't even come close.
This is caused by internet lag. It takes time for the data to go from your computer to texas, and then to the other players computer. So things are "off" due to that time difference. To get around this the game is designed to be played on your computer. So what you see is what is happening. So if you collide, you get damage. He may not see a collision, so he shouldn't get damage.
Best rule of thumb, AVOID RUNNING INTO OTHER THINGS. then you can only take damage when his bullets hit you.
