Playing a double you can’t cover is a risky move but it can work itself out. Sometimes you may have a double and have to play it because you're almost out of tiles or just have no choice. This is common near the end of the game as people increasingly draw from the boneyard. You can deploy a double you can't cover to force other players to cover it. If you know from the tiles on the board that most of the playable tiles are already used, this can force other players to open their trains and draw more tiles.