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Maidstone suffered a home defeat to Carshalton on the opening day of the Ryman League Premier Division season on Saturday.
United were behind on 16 minutes when a Charlie Ide capitalised on a defensive lapse by Jason Peacock.
It should have been 2-0 in the 32nd minute after Nathan Paul gave away a penalty but last season's top scorer in the division Richard Jolly blazed his spot-kick way over the bar.
But Carshalton did double their lead four minutes into the second half when Adrian Toppin brought down a clearance and sent a pearler of a shot high into Jamie Turner's top corner.
Meschach Nugent had the Stones's best chance with the score at 1-0 but he was denied by a fine stop from former Tonbridge keeper Matt Reed.
Tonbridge Angels 1 AFC Hornchurch 2
A goal six minutes from time condemned Tonbridge to a home defeat against AFC Hornchurch.
Joe Stevens put the visitors in front after just seven minutes when he found himself unmarked on the left side of the penalty area and made no mistake to beat keeper David Hogan, who was deputising for the suspended Lee Worgan.
The Angels equalised within four minutes, though, when Lee Minshull poked home after a goalmouth scramble.
Paul Booth came within a whisker of giving Tonbridge the lead on 70 minutes but his header from Steve Ferguson's fine cross shaved the crossbar - and AFC Hornchurch delivered the sucker punch with just six minutes to go when John Green was left all on his own to finish from the centre of the Angels box.