Here's a few of my personal choices...
First would be the infamous $20K Pyramid "Famous Losers" heartbreaker w/Didi Conn from 1979.
An eerily similar one occured on the $25K just 3 yrs later, during their 2nd wk on-air...the final WC category is "Things You Clip", and Jay Johnson offers partner Tom the clue "Something out of a newspaper" - a big no-no - as a clue, Tom gets it, and as they're celebrating his $25K win, the buzzer sounds, but they go to commercial before anything can be explained.
Upon returning, Dick says "You've prolly noticed that the mood around here is a bit more sedate than it was a moment ago...", even indirectly referencing the aforementioned Famous Losers incident, before playing a clip of Jay's illegal clue and explaining that it was no good and they had to take back Tom's $25K that he thought he'd won.
A $otC from 1984 where a contestant lost the cash jackpot after being beaten by only *$2*.
Any High Rollers ep where a contestant lost the Big Numbers by default via their leaving "1" as the very last number on the board.
And here's a near-heartbreaker: on Hit Man, champ Leon Reed (who'd previously appeared on Bullseye and TTD, maybe more) was asked the question "Who sang 'As Time Goes By' in Casablanca?" during the Triple Crown...he answered "Sam" (the character), but Peter's card read Dooley Wilson (the actor) and he was called incorrect, which only would've made a difference if he got to the final question in the $10K column...guess what question Peter was in the middle of reading when time ran out? :-(
However, good sense apparently prevailed...after deliberating during the break, Peter explained that Wolpert and crew had decided that Leon's earlier response of Sam on the ATGB question was acceptable, and he'd won the $10K as a result...he was quite delighted by this bit of news, as I recall. :-)
Chuck Donegan (The Illustrious "Chuckie Baby")