North is declarer in 4♥, and currently has 7 tricks. Here is the four card ending.
Declarer is on lead, and says...
"I'm playing Q♦ which will fell the Jack, and then just tables his last three cards T9♦ and 9♥ (implying all were winners) OK?"
Not statement is made about trump (eg no statement such as "I'll just play my ♦ and you can take your winner whenever you like"), so it is clear declarer is not aware there is a trump outstanding.
Neither is declarer aware that West is holding the boss trump rather than a small one.
Once the claim is contested North says happy to lose a trump trick.
TD is called and asked to adjudicate.
Assuming West doesn't ruff Q!d, and declarer thinks all his cards are good could TD suggest that he plays 9!h next. This would lose to T!h and when a spade is returned then EW score the last three tricks.
Declarer says it would be illogical for his to play the trump first as he can't be forced to play in "careless manner".
But given his counting and also his claim were both careless can he also be asked to play in careless manner?
Now that he is alerted to the fact that there is a trump outstanding then perhaps it would be logical for him to play 9!h in order to pull the remaining trump if he thinks it might be smaller than his? That way he would still make 11 tricks rather than ten!
You - as TD - have to decide how many tricks declarer is entitled to.
a) Allowing him to continue playing ♦ until West ruffs. Declarer can regain the lead and win remaining tricks. = 10 tricks.
b) Assuming West doesn't ruff Q♦, and declarer is now obliged to play his trump then he loses the final three tricks. = 8 tricks.
Which will it be?