Two that come to mind are Lucky Seven being lost on the first guess (guessed a 9, second digit was 1) and a no-horn loss in One Away when it was played for a Lincoln.
I've told this story many times and I'm still hoping to see this come up on YouTube, but I'll re-tell it.
It's 1982 and Lucky 7 is being played for a Jeep Renegade, a fairly nice vehicle and more expensive than most cars seen on the show. But, they haven't started playing Lucky 7 (or many other games, for that matter) for 5-digit cars yet. As Bob finished explaining the game, the contestant seems not to completely understand, and the dialogue between him and Bob goes something like this:
Contestant- "Oh, I guess it costs about...$11,000."
Bob- "OK...well, then what do you think the first number in the price is?"
Contestant- "A one!"
Bob- (looks at studio audience and then into the cameras) "I hate this. I hate to do this to him, but now I have to show him. He says one. What's the first number?"
The first number is revealed to be a nine. The contestant has lost on the first guess, for the first time ever...something I'm sure Bob made a point of mentioning just then.
Setting aside the fact that the contestant should have noticed that there were only spaces for 4 numbers on the board, would Bob really have been giving away too much information to say there are only 4 numbers in the price? A few years later, Bob would very frequently say "This car is worth over $10,000" or "There are 5 digits in the price of this car."