Crawley have now gone five games without a league win after a last-minute David Hunt own goal condemned them to a 1-0 defeat at home to Yeovil.
It looked like the game was heading for a draw until substitute Hunt nodded Jamie McAllister's corner into his own net.
It was the first time in 14 league games Crawley had failed to score and, while they have had only three defeats in the last 11, they have not won in League One since beating MK Dons a month ago.
A lively start saw Paul Jones save well from Keanu Marsh-Brown on three minutes and Crawley awarded a free-kick rather than a penalty for a foul on Nicky Adams on the edge of the box moments later.
In the 21st minute Marek Stech spilled Mike Jones' shot but recovered to push away Billy Clarke's follow-up from a tight angle.
At the other end Jones saved a Joe Edwards shot at his near post and made an even better stop to keep out Paddy Madden's header with his legs on 37 minutes.
Crawley's defensive problems increased just before half-time when Kyle McFadzean went off injured which meant a reshuffle with Hunt coming on at right-back, Nathan Byrne switching to the left and Sadler moving to the middle.
The game waned in the second half when chances were at a premium.
Crawley had the better of them with a goalbound shot from Mark Connolly from an Nicky Adams corner blocked on 66 minutes and then Josh Simpson's shot being cleared off the line by Luke Ayling seven minutes later.
Gary Alexander's header also hit the top of the crossbar.
The goal eventually came at the wrong end in the final minute as Hunt headed a Jamie McAllister corner from the right into his own net.
For reaction, analysis and pictures see The Argus tomorrow.