Saboteur can mark planets as being booby-trapped. Whenever any players land ships on a planet, Saboteur must reveal if it has marked it as booby-trapped. If it is, then all ships on the planet (those that were already there as well as those that just landed on it) are sent to the warp.
In previous editions, Saboteur had to mark which planets were trapped with pencil and paper. In the current edition, the expansion that includes Saboteur also includes special tokens, marked on one side as traps or decoys, that it places near planets to mark them as trapped.