Humbled at already-relegated Portsmouth on Saturday, Chris Morgan's men were undone by a 49th-minute goal from Chuks Aneke.
The Blades had already booked their play-off place but Aneke's winner means they have won just one of their last seven games.
Aneke stunned the visitors four minutes after the break with a superb 25-yard finish.
An early goal from midfielder Mike Jones gave Crawley a deserved 1-0 victory and condemned Preston to their first defeat in five games.
Jones drove Crawley ahead after only six minutes with a bouncing right-foot shot from 25 yards into the bottom corner.
Joe Jacobson's classy free-kick wrapped up a deserved 1-0 win for Shrewsbury against former club Oldham.
Jacobson, the stand-in captain, chose a perfect time to grab his first goal of the season when he whipped a 20-yard effort over the Oldham wall and high into the net four minutes into the second half.
Both of these sides guaranteed League One survival last weekend.