Charlie Allen – son of Bees boss Martin – scored as Margate overcame Barnet in Kent, despite the Bees having the better of the contest for long periods.
Allen gave Margate the lead after 12 minutes, darting into the penalty area to prod beyond Graham Stack.
Charlie MacDonald levelled in the 62nd minute, his third goal against former employers in pre-season after notching against both Stevenage and Brentford. Eight minutes later, Anthony Riviere curled home to secure the win for Margate.
Gavin Mahon was the only player named in Barnet’s starting XI who was not contracted to the Bees, though the midfield general remains in talks with Allen about a deal for the forthcoming season.
Summer signing John Akinde skippered the side as the rotation of the captaincy continued.
For hosts Margate, former AFC Wimbledon forward Luke Moore started on the bench but former Wycombe goalkeeper Nikki Bull, ex-Brighton defender Sam Rents and Brett Johnson, formerly of the Dons, ensured there was plenty of Football League experience in the side.
The Bees had begun brightly enough, earning two early corners and seeing a Luisma Villa shot blocked before falling behind after twelve minutes.
Allen Jnr, who has previously played for his dad at both Gillingham and Notts County, got the opener when he tucked beyond Stack after finding himself in the right place at the right time.
Eight minutes later, the Ryman Premier League side ought to have grabbed a second when Jason Prior connected with a Lewis Taylor cross, nodding just wide for Terry Brown’s side.
Barnet had their first notable chance after 32 minutes when Gate goalkeeper Bull dashed off his line and clattered the onrushing Keanu Marsh-Brown on the edge of the penalty area.
From the resulting set piece, Bull redeemed himself when he got down well to prevent Elliott Johnson’s shot going in.
Bull then prevented the Bees from levelling once more, blocking well with his feet from Marsh-Brown before Akinde volleyed the rebound over the crossbar.
The hosts threatened twice before the break, with Zac Attwood the source on both occasions.
First the forward saw his header pushed over the bar by the alert Stack, before sending a shot narrowly over and onto the roof of the net.
Half-time saw four Bees changes, with the unexpected introduction of trialists Lee Cook and Sam Togwell – most recently of Chesterfield – in place of Marsh-Brown and Luisma.
Having been in the ascendency for much of the second half, the Bees levelled just after the hour mark.
Akinde flicked on a long ball, allowing MacDonald to control on his chest before poking past Bull to level for Barnet.
Five minutes after playing his part in the equaliser, Akinde could have put the visitors ahead, bringing the best out of Bull with a fine header from close-range. The former Brentford loanee then kept out the follow-up to keep the scores level.
Tom Phipp tried his luck soon after, sending an acrobatic effort narrowly wide of Stack’s post.
In the 70th minute, Barnet fell behind to a sucker-punch, having continued to control proceedings following their equaliser.
Gate full-back Riviere was allowed to progress, and taking advantage of the opportunity to get forward, curled beyond Stack to put the Kent side back in front.
The Bees continued to push for an equaliser after falling behind but could not find a way past the in-form Bull, who was named the matchday sponsor’s man-of-the-match for his part in keeping Barnet at bay.
Barnet: Stack; Yiadom, N'Gala, Saville (Stephens HT), Johnson (Muggleton HT); Marsh-Brown (Togwell HT) (T), Mahon, Weston (Vilhete 71), Luisma (Cook HT) (T); MacDonald (Lowe 65) & Akinde (c).
Subs not used: Gambin, Sykes, Nurse & Cowler (GK).
Margate: Bull, Riviere, Rents (Bennett HT), Allen (c), Beales (Wassmer HT), Johnson (Stuart HT), Phipp, Wills, Taylor (Moore 56), Attwood, Prior (Moss 56).
Subs not used: Beadle, Donnelly Attendance: 468 Referee: