Ben Tozer's last-gasp strike condemned Barnet to a 2-1 defeat, and completed a second-half turnaround for Northampton.
Tozer arrived on cue in the 89th minute to thump the ball past Dean Brill after Adebayo Akinfenwa had held up Marlon Jackson's cross in the box.
Ricky Holmes put the hosts ahead after 17 minutes but they were pegged back six minutes after the break when Arron Davies tapped home, and Tozer completed the comeback.
Holmes put Barnet ahead when Kelvin Langmead sliced Mark Marshall's cross straight to him on the edge of the box.
But the hosts surrendered their lead six minutes after the break when a high ball was flicked across the face of goal by Akinfenwa and Davies charged in to tap home from point-blank range and Tozer finished the job.
The Bees had a lucky escape inside ten minutes when captain Mark Hughes carelessly lost possession before stopper Dean Brill fumbled Davies' resulting 25-yard drive.
Holmes struck the post just before his goal and Sanchez's side should have re-taken the lead shortly after Davies' goal, only for Charlie Taylor to blaze over the bar.
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