ACBL. Club game. Match points. NS C players. EW advanced A players. W led 8♦ which S won. S returned J♣. W sensed what was going on and rose with the A, followed by small ♣ from W and dummy and 3♠ from S. We got it sorted out - I ruled that it was a fifth card played to a trick as it was clear through her action and a somewhat incoherent conversation that she was quite eager to ruff a ♣. Since W had won the trick, he took advantage of the free finesse and cleared the trump suit. As I was walking away, W made a comment about there being UI, which I didn't consider at the time but I have thought about a good bit since.
Let's say W had ducked the ♣ hoping to set up three tricks in that suit. Given the early stage of the play and nothing being contributed by the defenders during the auction, declarer might not have worked out yet that the J♣ was stiff. Now N rises with the K, E plays low and S contributes a trump as the fifth card to the trick. Now we have an exposed card, P is on lead and the best declarer can do is prohibit the lead of a ♠ under L50D.2.(a). N is happy to comply by leading a ♣ which S ruffs, disposing of a major penalty card at its first legal opportunity. It would be a better solution for declarer if I could rule that the ♠ was led (rather than being a fifth card to the trick), which would again give declarer a free finesse of the trump suit. Since the facts and circumstances didn't support such a ruling, declarer gets a bad result. About the only relief I can conjure up is to award an adjusted score under L12. I just had not considered that there could be UI resulting from such an action.
I guess that's why we want experienced TDs - people who have been around long enough to have seen everything.