Lett was a talented player and a cornerstone of the Cowboys defense during his tenure, but fans likely remember him for his infamous plays. Two of the top three of ESPN's "25 Biggest Sports Blunders" are attributed to Lett. The fans ranked him #1 and #3, whereas an expert panel placed him at #2 and #3.
The first play (ranked #1 in the ESPN fan list, #2 in the ESPN expert panel) occurred in January 1993, in Super Bowl XXVII. Late in the fourth quarter, Lett recovered a fumble on Buffalo's 45 yard line and ran it back towards the endzone. When he reached the 10 yard line, he slowed, and held the ball out as he approached the goal line. However, he didn't see a hustling Don Beebe, chasing him down from behind. Beebe knocked the ball out of Lett's outstretched hand just before he crossed the goal line, which sent the ball through the endzone, and resulted in a touchback that cost Lett his touchdown. Lett later said he was watching the Jumbotron, and trying to do a "Michael Irvin", where he put the ball out across the goal line.
The Cowboys had a commanding 52–17 lead at the time, and the play did not affect the outcome of the game, but it certainly embarrassed Lett, and it is still well known by football fans today. Lett's gaffe also cost the Cowboys the record for most points scored in a Super Bowl (55, by the San Francisco 49ers in Super Bowl XXIV).
The second play (ranked #3 in both ESPN lists) occurred during the very next season and was actually more serious as it resulted in a Cowboy defeat. On Thanksgiving Day in 1993, during a rare snow and sleet storm in Dallas, the Cowboys, who came into the game with a 7-3 record, were leading the 8-2 Miami Dolphins 14–13 with 15 seconds remaining in the game. The Dolphins attempted a 41-yard field goal to take the lead but the kick was blocked. While most of his teammates began celebrating, Lett attempted to recover the ball. He slipped on the ice as he tried to pick up the football, and Miami recovered the "muff" on the Dallas one yard line. Had Lett simply done nothing, the Cowboys would have automatically received possession and could have run out the clock. By touching the ball and then failing to hold onto it, Lett enabled the Dolphins to take possession and then try another field goal with 3 seconds left on the clock. This second attempt was successful and the Dolphins won the game 16–14 as the clock expired. The play actually didn't hurt the Cowboys' season as they won all of their remaining regular-season games and went on to win the Super Bowl, whereas the Dolphins (who took over sole possession of the lead in the AFC East with the win) would not win another game for the rest of the year, dropping their last five games and finishing out of the playoffs. In 2008, the game was named the third-most memorable in the history of Texas Stadium by ESPN.