[Pierre-Marie Baty]

Lag as you say shouldn't cause any problem but for the client. The only machine on which the game really happens is the server. If the server updates its clients correctly, then all clients will be in the exact same state. Bear in mind that lag also means a time shift, hence it's not ONE entity which is out of sync but the WHOLE client pack which is late about the server. But minus that time shift all the entities and the forces applied to them should be in the exact same state, which is why accurate client-side prediction is more than possible, it should be a must for such games.

*edit*
If you want me to be more accurate in the answer, I'll just say that in fact what can cause the problems you are talking about is not the lag, but any attempt of lag compensation.