Crawley capitalised on defensive errors to defeat Sheffield United and dent the Blades' automatic promotion hopes.
The hosts hit the woodwork either side of half-time through efforts from Ryan Flynn and captain Michael Doyle.
And they were made to pay when Paul Hayes robbed United defender Neill Collins to allow former Blades loanee Billy Clarke to fire home.
Hayes then seized upon Harry Maguire's hesitation on the edge of the box to slide home the second and seal victory.
The result leaves Sheffield United six points off second-placed Bournemouth with two games in hand.
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Sheffield United manager Danny Wilson told BBC Radio Sheffield:
"We've been done by a couple of mistakes, from two boys in particular that have been outstanding for us the last couple of seasons.
"There were good chances in the game, hitting the post. We've been quite unlucky in that respect following things through - not getting the ball in the back of the net has been our problem recently as we well know.
"The saving grace - we've had a few other results that have gone our way and they've done the same as us, so all it does is knock us down by a game.
"Lets hope if we've have had any small mercies it'll be the same for the next day or two."
Crawley Town manager Richie Barker told BBC Sussex:
"It's a proud moment for me, but a fantastic achievement for the players and the football club and thoroughly deserved I thought tonight.
"They're big, they're strong and have a lot of experience and put us under pressure, and I thought our two centre-halves handled it extremely well.
"Sometimes when you play against a team who are going to be big and strong, you then get sucked into doing the same thing back but to be fair, we didn't and passed it very well."