An old technique used on some Renegade Multiplayer maps, "tunnel beaconing" often resulted in kicks or bans on the server. The technique: some maps have tunnels close to enemy structures. All one has to do is place a Beacon. The other side won't expect a Beacon outside of their base (and often can't find it), and the beacon destroys the structure. This is ruled as unfair, as there is no known counter, and if a team decides to do it as a whole, impossible to stop.
An example of this strategy can be easily employed in the Skirmish ("Multiplayer Practice") map, especially if playing as Nod- go through the "back entrance" to the area near the other side's Power Plant, and plant a beacon behind the "door frame" of the entrance.