He didn't stay terrible, but the single worst performance I've ever seen in an NFL game comes with a conspiracy theory attached. I'm actually just going to repost what I said last time, since getting the details right is a job of work.
In 1997, the Colts were terrible, but it wasn't because their QB play was terrible. Jim Harbaugh had a solid season, and backup Paul Justin wasn't bad when called upon. In their final game of the season against the Vikings, Justin wasn't available, and Harbaugh got hurt in the second quarter and disappeared into the tunnel to the locker room. In came the Colts third-stringer, an undrafted second-year guy who had been forced into starting one game earlier in the season, and had thrown three picks.
It didn't get better in this game. Entering a game that was tied 10-10, the third-stringer committed three straight turnovers, two interceptions and a fumbled snap, leading to center Jay Leewenberg practicing snaps with him on the sidelines. The Vikings turned those three turnovers into 19 points (two missed two-point conversion attempts) and led 29-10 at the half.
At the beginning of the second half, Jim Harbaugh came running back down the tunnel, ready to re-enter the game. With 8:25 to go, Harbaugh hit Marvin Harrison for a TD on 4th and goal from the 2, to bring the Colts back to within 8 points. Unfortunately for him, he was injured on the play and had to leave, this time for good.
The Colts got the ball back, and had a chance to drive for the tying score...but apparently that snap practice from earlier hadn't helped, since the third-stringer again dropped the ball, nose tackle Jerry Ball recovered, and the Vikings kicked a FG to take an 11 point lead.
Just to finish off his performance, our hero managed to throw one final INT to ice the game. His final line was 5/8 for 29 yards, 0 TDs and 3 INTs. Plus the two fumbles lost.
You'd think that after this performance, a guy would find himself needing to find gainful employment in another industry. But no. Our hapless friend stuck around in the NFL for more than a decade. In 2002, he started a playoff game for the Cleveland Browns, and played quite well. His name? Albert Einstein Kelly Holcomb.
Conspiracy theory version? The Colts needed a loss in order to hang onto the top pick in the draft. Holcomb was a double agent, sabotaging the Colts on the field, while team officials kept Harbaugh in the locker room for "tests". In return, he was kept on the payroll for another two seasons, backing up new QB Peyton Manning, and earning a healthy paycheck.