[dogscoff]

The other mine exploit (well, more of a tactic than an exploit IMO) is for player one to lay a huge stack of mines at a location, then gift the minesweeper to player 2 who then lays a bunch more. Assuming player three is at war with both players 1 & 2, the mine-per-sector limit has been effectively doubled.

And of course you can do this as many times as you have allies...