Frank de Boer's Ajax side appeared to be heading for a fourth league game without a win when they were pegged back at 1-1 with time running out at De Adelaarshorst..
However, Van der Hart - who is on a season-long loan from Ajax - failed to connect when he attempted to clear Wesley Verhoek's back-pass and could only watch as the ball trickled into the goal.
Anwar El Ghazi put Ajax ahead in 24th minute when he tapped home at the back post to complete a flowing move from the Dutch champions.
It appeared as though 15th-placed Go Ahead would claim a much-welcome point in the when Sjoerd Overgoor let fly with a stunning left-foot shot from 25-yards that nestled in the left-hand corner 14 minutes from time.
But Van der Hart's error in the 88th minute denied Foeke Booy's men a share of the spoils and ensured Ajax are 12 points behind leaders PSV.
Goals either side of half-time from Karim El Ahmadi and Colin Kazim-Richards helped Feyenoord to a 2-1 victory over Cambuur, having suffered a defeat against Heerenveen last weekend.
The visitors pulled a goal back through Martijn Barto just after the hour despite having Bartholomew Ogbeche sent off, but Cambuur have now lost five in a row in all competitions.
Groningen suffered a first home league defeat since last March after surrendering a 2-0 half-time lead to go down 4-2 against AZ.
Erwin van de Looi's side appeared well in control as an own-goal from Steven Berghuis and Michael de Leeuw's strike put them in control.
However, the hosts capitulated after the break - Berghuis scoring at the right end to start the comeback, before Jeffrey Gouweleeuw struck and a brace from Markus Henriksen sent AZ up to fourth.
Elsewhere, struggling Heracles' winless run stretched to nine matches as they suffered a 3-0 loss at Willem II.
