3. WR Tyrell Williams
After showing some promise with the Chargers and the Raiders (15 or more yards per catch in four straight seasons from 2016-2019), and after missing the 2020 season due to a torn labrum in his shoulder, Williams got a theoretical opportunity for a big role in Detroit when Holmes signed him to a one-year deal in 2021.
Then Williams suffered a concussion in Week 1 against the 49ers, and after being out awhile he was waived from IR with an injury settlement in November. So in theory he was healthy enough to play in November 2021, having presumably cleared concussion protocol, but the Lions apparently did not plan to put him on the 53-man roster and he may have asked to be let go. He was not claimed by another team, and he has been out of the league the last two seasons.
All told Williams played 39 snaps and had two catches for 14 yards in a Lions' uniform. It's tough to call a one-year deal a big mistake, and Williams fell short due to an injury circumstance out of his control. But expectations were naturally more than what came to be, and even when it appeared he was healthy enough to play again the Lions decided they were done with him.