When players collide with ex. walls, they appear to run through them. The server also thinks the player run through the wall, which means that when the player reconnects, he will spawn at the desynced position. You can use this to breach through walls and enter otherwise unreachable places.
The desync also affects fire fights. You have literaly no real sense of where your character really are. If I run inside a building and collide with a few walls, the server and other player will see me on the outside, and they will be able to kill me.