A bad beat is when a bettor has a wager that is seemingly decided as a winner, then in the last few moments, the switch gets flipped in an unseen way to change the projected winning bet into a losing one.
There is no better example of this, than Super Bowl 51. The Atlanta Falcons were +3 going into the game against the New England Patriots. With 2 minutes left in the third quarter, the Falcons held a 28-3 lead. All of the bettors that backed the Falcons, your +3 tickets are all but won, until it wasn't.
The Patriots in the final 17:06 minutes of the game went on to score 25 unanswered points to close out the game. This scoring run included the team converting both of their two-point conversion attempts. When the clock hit zero, you had a 28-28 score. However, Falcons bettors had a chance to win their bets in overtime. At the very least, they were hoping to push with a Patriots field goal.
The Patriots proceeded to win the coin toss, elected receive and scored a touchdown on the opening possession to win by 6 points. That touchdown capped off one of the most miraculous comebacks in sports history. It also broke the hearts of many bettors. This game is one of the worst bad beats that has ever occurred. It also is the perfect example that you can never count your bet as a win until the game is over.
Whether it is a 25-point comeback in the Super Bowl or a meaningless fumble recovery touchdown at the end of a game, improbable events can happen at any point. If you gamble on sports enough you will, unfortunately, have to deal with some bad beats.