I appreciate all the comments - especially to Xiao for a very thoughtful reply. However, I am now more than ever confused about UI resulting from "partner's misinterpretation" of a bid. L75A cautions the offender to avoid taking advantage and bidding on (if appropriate) as though the erroneous call had been correct. In my limited experience, this is rarely done. Offenders or their partners try to "fix it" with the least amount of damage and I've never seen a NOS object. Several examples come to mind.
Example 1:
I sat N and announced my partner's point range (15-17). I inquired about W's call and was informed that it was natural. Thinking systems were off at that level, I bid to show my
♦ suit and game-going values. My partner immediately announced "transfer", which E was delighted to hear but had to know was improbable. My partner dutifully bid accordingly. The question now becomes, who made the error? Since we had no agreement for 3-level overcalls of 1NT openings, I must conclude that it was a mistaken explanation. However, if my partner believed it to be correct I would be reluctant to correct it under L20F5(b). As I read L75A and L16B1(a), I must conclude that I should bid as though my hand contained 5
♥, which would require me to bid 3NT, my partner correcting to 4
♥ as appropriate. My next bid could reasonably be interpreted by partner as some extension of Smolen (which we don't play), but was delighted to raise me to game. Our opponents did not object to the auction and I did not correct the transfer announcement. Did "transfer" provide me with UI which I then used? Could I make the case that I ignored her "transfer" and only looked at her bid card, assumed that she meant she had a
♥ stopper, I then bid
♠ to show a holding there, and she bid the
♠ game based on the auction?
Example 2:
Sorry I couldn't find the hand record. P opened 1
♦ and I bid 1
♠. P bid 1NT. I bid 4
♣ (Gerber, and clearly marked). My LHO asked what my bid was and P told him it was a splinter in support of
♠ (???). P then bid 4
♠, which I raised to 6NT. Again, as in the last example, I could make a case that I ignored P's explanation, concluded from his bid he held two aces (although I held 3) and bid the slam. In reality, I figured he got it wrong and took my chances with a tremendous hand. I did call the TD and explain our agreement, which both opponents knew to be the case. No one suggested the auction be reopened nor did the opponents claim damage at the end of the play - we made 7.
Example 3:
I overcalled a 1
♣ opening with 1NT. After a pass, my P bid 4
♦, which I forgot was a transfer to
♥. After concluding he must hold a long
♦ suit, I raised to 5. He then bid 5
♥ and the veil was lifted. I advised our opponents that was a transfer which I missed and we were off 1, most other tables bidding and making 4. Should my P have been allowed to bid? My failure to announce "transfer" was UI to him. I expect playing 5
♦ would have given us a worse result than 5
♥.
There is much discussion about UI and LA in bridge land. I know it's all facts and circumstances, but it seems from the examples above that "fixing" an mistaken call is frequently done and rarely opposed. However, is that the right answer?