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Frannie Collin scored an injury-time winner as Maidstone moved five points clear at the top of the Ryman League Premier Division on Tuesday night.
He got in front of keeper Charlie Horlock to meet Alex Flisher's cross in the fourth of seven added minutes at the Gallagher Stadium.
United played poorly in the first half and trailed to a stunning 25-yard free-kick from Harrow's Danny May after 31 minutes following a foul by Steve Watt.
Boss Jay Saunders laid into his players at the break and made a double change with Jay May replacing Ben Greenhalgh and Jamie Coyle on for groin injury victim Watt.
May went up front with Flisher moving wide-left and the pair led United's fightback.
Stones had two strong penalty appeals turned down, both involving May.
He appeared to be tripped by Francis Otira as he worked room for a shot and seconds later saw a strike diverted wide by a Harrow hand.
Maidstone pressure finally paid off with the equaliser on 79 minutes, substitute Ashley Miller collecting Collin's pass and crossing to the far post for in-form Flisher to finish.
The visitors' time-wasting frustrated Stones fans and players but Harrow's antics would come back to haunt them with referee David Spain adding on seven minutes.
And the Gallagher Stadium went wild when Flisher's near-post cross set up Collin.
Maidstone: Worgan, Simpson, Mills, Watt (Coyle 46mins), Miles, Rogers, Bodkin (Miller 74mins), Parkinson, Greenhalgh (May 46mins), Collin, Flisher. Subs not used: Brown, Godmon.
Attendance: 1,347