In general, any time a contestant sweeps an entire category, which is usually followed by applause.
Prior to Season 20, any contestant who retired as an undefeated five-time champion.
From Season 14-19, all undefeated champions also won a car.
In general, any of these cases where a contestant bets a true Daily Double near the end of Double Jeopardy! (which takes balls) and provides a correct response:
A trailing contestant with less than half the 1st place score successfully narrows the gap between 1st and 2nd place, preventing a "lock" game.
A trailing contestant will a little more than half the 1st place score steals the lead.
A leading contestant increases their lead and wins with a "lock" game because of their true Daily Double.
Fall 1985: Chuck Forrest, widely considered the first truly great Jeopardy! player. No less than Ken Jennings has remarked that, while he was a fan of the Trebek Jeopardy! since day one, Chuck Forrest's run is his first really clear memory of the show. This was because Chuck was a Season 2 undefeated champion who:
Won all of his non-tournament games in lock or lock-tie fashion — in other words, going into Final Jeopardy!, he couldn't lose unless he got Final Jeopardy! wrong and made a completely logic-defying wager (or, as they say, pulling a Clavin).
Set what was then the show's 5-day winnings record...at the end of his fourth game,
And was so dominant in his Tournament of Champions, specifically the finals, that he single-handedly forced the producers to institute a minimum winnings guarantee for ToC finalists. To elaborate, the winner of the ToC that season won $100,000; the other two finalists' winnings were "what you won during the two finals games"; and losing in the semifinals that year got you $5000. Chuck's opponents' totals for the two finals games were $4000 and $3000, or less than they would have gotten had they lost in the semifinals. They appealed and were given $5000, which led to subsequent ToCs having a minimum guarantee for the two runners-up.
June 1987: Bob Verini, then a playwright and actor from New York City. Best known for his impersonations, particularly Julia Child, but was also a superb player. He cashed in with $46,802 in his original five appearances and won the Tournament of Champions that fall. He also finished second in Super Jeopardy! and placed third in the Million Dollar Masters tournament.
Bob's victory in the '87 Tournament of Champions in a Moment of Awesome in itself for three reasons. One—Going into the second game, he was in third place. Two—He came up with the correct response in Final Jeopardy! after changing it midway. Here's the Final Jeopardy! clue and Bob's response. (The clue was "He said, 'I am the last President of the United States'." Bob wrote down Franklin Pierce, then crossed it out and replaced it with the correct one—James Buchanan.) Third—He wagered just enough to win the tournament by a single dollar!
January 1990: Frank Spangenberg, then a member of the New York Transit Police Department (now the Transit Bureau of the New York City Police Department), set a record of $102,597 during his five days on the show, a record that lasted for 13 years. In addition to that, he set a then-one day record of $30,600 on his fifth and final day.
At the time, there was a $75,000 winnings cap, so Frank had to donate $27,597 of his winnings to Gift of Love Hospice, a facility operated by the Missionaries of Charity. In a moment that also doubles as a Heartwarming Moment, his money was used to bring the place up to fire code.
Frank later went on to win the Tenth Anniversary Tournament, and made appearances in the Million Dollar Masters and the Ultimate Tournament of Champions.
October 20-26, 1999: Eddie Timanus, the first blind contestant to compete on the show; he was expertly accommodated by the show — he had a Braille keyboard at his podium to type Final Jeopardy! responses, and was given a Braille card containing the categories by Alex before each round (this was shown on-camera, although it was up to Eddie to recall what clues were remaining on the board). Merely competing on the show with such a...well, some would consider it a handicap or disadvantage...would have been awesome on its own, but Eddie was a great sport about the whole matter and was very smart — he became an undefeated champion!
June-November 2004: Ken Jennings' miracle run. He won seventy-four straight games and won over two million dollars. Four facts to put this in perspective:
Before he started his run, the record-holder had been on for eight straight days and had only won about $200,000. In fairness, though, up until 2003 (about a year before Jennings's appearance), 5-time champions were retired from the show undefeated, so this by itself doesn't tell the whole story.
More amazingly, in nine years of post-Jennings play, only three other people (David Madden with 19 wins in 2005, Arthur Chu with 11 wins in 2014, and Julia Collins) have even made it into double-digit non-Tournament victories, and until Julia Collins in mid-2014, none have gotten to 20 straight.
A typical winning score on a non-tournament episode of Jeopardy! is around $15,000 (give or take a couple thousand - below $10,000 or above $20,000 are both fairly uncommon). Ken Jennings averaged just over $34,000 per non-tournament game. He also tied the then-one day record of $52,000 three times before finally making a big enough wager to exceed it by $23,000 in the season 20 finale.
65 of Ken's 75 games were won in lock fashion — in other words, over 85 percent of his games had him dominating his opponents so thoroughly that he was guaranteed to win no matter what he or his opponents answered/wagered during Final Jeopardy! unless he pulled a Clavin. Additionally, before his 75th game, he had only one game (his 5th) in which he would have lost in Final Jeopardy! if his closest opponent had gotten their answer right.
Ken's 18th game is also notable. He had $28,200 at the end of Double Jeopardy!, but his opponent was not that far behind with $24,400 thanks to a true Daily Double. Both got the correct Final Jeopardy! response, and Ken's opponent finished with a very high second place total of $44,400. If only there was an exception to the 1984 rule patch...
May 2005: Brad Rutter owning Ken Jennings in the Ultimate Tournament of Champions, getting almost twice as much money as Ken Jennings and temporarily claiming the record for largest game show winner in history with over $3 Million.
March 16, 2007: With his two opponents tied at $8,000 and within reach, Scott Weiss figured they would risk it all. So he bet for a tie instead of the win, which led to the show's first-ever nonzero three-way tie and a very interesting intro by Johnny Gilbert on the next episode.
And as Alex explained in that next episode, Scott did that because he overheard a boy asking Alex before Final Jeopardy! if a three way tie had ever happened. Alex told him no, so Scott set up the wager hoping it'd force the tie. Everybody wins!
September 13-16, 2010: Season 27 began with a runaway win by Roger Craig, who also posted a total runaway in the next game — setting a new one-day record of $77,000 (beating a record set, incidentally, by Ken Jennings) in the process.
Roger's third and fourth games were quite impressive, too. On the third, he took a $10,000 tumble on a Daily Double and finished Double Jeopardy! only $1,600 ahead of second place. Luckily, he was the only one to get Final Jeopardy! right. The fourth day, another contestant held a significant lead over him in the first half, but Roger quickly jumped back into the lead during Double Jeopardy!, including a True Daily Double.
That may be so, but Watson GUESSED on a Daily Double with 32% confidence and got it RIGHT. Not only that, but Watson actually made the wager on that day's Final Jeopardy himself! (He got it wrong, but wagered only $947!)
Ken Jennings' Final Jeopardy card: "I for one welcome our new computer overlords".
November 14, 2011: In the final match of the Tournament of Champions, Roger Craig hunts down two Daily Doubles, one after the other. He bets all of his money on both of them, and gets the answers right; effectively quadrupling his score.
December 16, 2011: Jason Keller had been auditioning for the game for 16 years; he remained champion on Jeopardy! for nine days, winning $213,900.
February 12, 2013: Leonard Cooper wins the Teen Tournament despite being a wild card non-winner in the quarterfinals… helped in no small part by a ballsy $18,000 (yes, eighteen thousand) wager late in Double Jeopardy! on the final game of the tournament.
July 31, 2013: During a Kids' Week game, Skyler Homback, a 12-year-old from Kentucky, amasses an impressive $36,600 heading into Final Jeopardy!, while his opponents have scores of $9,600 and $6,400. The Final Jeopardy! category is "The Civil War", Skyler, who is deeply interested in this topic, wagers a hearty $30,000note (a wager of $17,399 or less would have guaranteed a win had Skyler been caught off-guard). Skyler gives a correct response, finishing with a grand total of $66,600 — the third-highest single-day score in the show's history, standing only behind Roger Craig (with $77,000) and Ken Jennings (with $75,000).
2014: The entire "Battle of the Decades" tournament, where contestants from the 1980s onward competed against each other. Luminaries such as Chuck Forrest, Eddie Timanus, and Ken Jennings were among the competitors. The three finalists on the last two episodes of the tournament? Ken Jennings, Brad Rutter, and Roger Craig.
And the ultimate winner? Brad Rutter, keeping his record of never being defeated by a human opponent intact, beating Ken Jennings for the second time, and passing Jennings to retake the lead for all-time game show winnings. Not only that, but he was actually in last place for a majority of both final matches before mounting a comeback both times midway through Double Jeopardy!
In a subversion, Rutter was reduced to being a spectator in Final Jeopardy in the 2nd game. Rutter brought a $3000 lead over Jennings from game 1, while Jennings had an $1800 lead over Rutter going into Final Jeopardy in game 2. Craig couldn't catch either of them, so the outcome of the Tournament rested entirely on whether or not Ken Jennings would answer the Final Jeopardy clue correctly. If he did, Brad Rutter's lead from the first game couldn't hold up, even if Brad doubled up in Final Jeopardy, and if he missed, Brad would win regardless of what he did. In recognition of this fact, Brad Rutter's wager during Final Jeopardy was $0.
May-June 2014: Julia Collins' hot streak of 20 games. By her 11th game, she became the longest-running female champion in the show's history, and just kept rolling. Most of her games have been "locks" as well, but game 20 became even more impressive as she salvaged the win through a narrow lead.