When Backgammon with doubling cube was still in alpha or beta and was called backgammon matchplay, I filed a bug report for it. See
https://boardgamearena.com/bug?id=103036.
But since nobody upvoted it then and since it seemed to be a minor issue my bug report got ignored or was just never seen.
I agree with Elnad that this is an issue and should be corrected. If you write a bug report for the game backgammon please mention it here and I will upvote it.
I quote from my bug report with title "DEAD CUBES: A player cannot double beyond the number of points still needed to win a match.":
A cube is called dead for a player if the current state of the cube is enough to win the match. It is not allowed to double a dead cube - not even by accident! If a dead cube is doubled this has to be undone in a tournament.
See the official rules of the World Backgammon Association:
www.world-backgammon-association.com/general-rules
4.4. 7) DEAD CUBES: A player cannot double beyond the number of points still needed to win a match.
In the game 433986641, move 287, white is leading 6-4 in a game to 7. Red has previously doubled so the cube is on white's side. The cube should be dead (i.e. greyed out) since doubling by white is an illegal move.
The implementation, however, is allowing to double dead cubes! This is against the rules of Backgammon.
You might say that this is not important because nobody will double anyways in such a situation. But in live tournament this happens every now and then. As soon as the player notices the mistake (this can be many moves or even games later), the doubling is undone (this is according to tournament rules, even finished games can turn into unfinished games and need to be continued).
Since this undo-feature of illegal moves is not available on BGA the implementation should take care that no illegal moves can happen.
