Eagles striker Wesley Verhoek was credited with the 88th minute goal after 20-year-old Van der Hart's calamitous error gifted the visitors a controversial win that puts second-placed Ajax on 46 points from 22 games, 12 behind PSV Eindhoven.
The game seemed headed for a 1-1 draw when Verhoek produced a measured back pass from 35 metres but when Van der Hart attempted a clearance he completely missed the ball which nestled into the bottom corner of the net.
It was a deeply embarrassing moment for the young stopper against his parent club and cost his current team a precious point in their relegation battle with the Eagles remaining one place and three points above the drop zone.
Anwar El Ghazi had given Ajax the lead midway through the first half before Sjoerd Overgoor equalised in the 76th minute with his first goal in 2-1/2 years.
Feyenoord moved up to third with a 2-1 home win over 10-man SC Cambuur as goals either side of halftime from Karim El Ahmadi and Colin Kazim-Richards secured the points.
They had to hold on though as, after Bartholomew Ogbeche was sent off for the visitors in the 57th, Cambuur pulled one back through substitute Martijn Barto five minutes later.
Mid-table Willem II defeated third-bottom Heracles 3-0 with strikes from Samuel Armenteros against his former club, Robbie Haemhouts and a Jeroen Veldmate own goal.
AZ Alkmaar came back from two goals down to defeat hosts Groningen 4-2 and move up to fourth in the table.
AZ midfielder Steven Berghuis was the third unfortunate player to put through his own net in the Dutch league on Sunday before Michael de Leeuw doubled Groningen's advantage.
Berghuis made up for his earlier error by scoring at the right end in first half injury-time, before an assault on the home goal in the second period saw AZ pull clear.
They netted again through Jeffrey Gouweleeuw and a double from Norwegian midfielder Markus Henriksen.
(Reporting by Nick Said in Cape Town; Editing by Ken Ferris)
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