When Danny Forrest added to Anthony Griffith's first half opener seven minutes after the break, the Shaymen were cruising.
It appeared to be a case of just how many they were going to score.
Then they imploded spectacularly, allowing York three goals in a 12-minute spell with former Town frontman Alex Meechan bagging what proved to be the winner.
There had been moments of hesitation at the back in the first half from Town, but nothing to suggest such an incredible collapse.
And it is something they are clearly going to have to rectify if their season is to go anywhere.
The opening stages of the Yorkshire derby were typical for such a game, frantic and without much shape.
Onome Sodje, who later scored the goal that sparked the City fightback, should have put the home side ahead after 10 minutes when he raced clear only to hit the bar with the goal at his mercy.
But it was the Shaymen who broke the deadlock after 16 minutes with Griffith's second goal in nine matches.
It was a little fortunate as the loan midfielder's shot, from Jake Wright's half cleared cross from the left, swerved in front of York keeper Stephen Henderson and slipped through his grasp.
Any good fortune they had on that occasion was absent when striker Jon Shaw was forced to leave the field with a dead leg just four minutes later.
And the home side enjoyed a good little spell after that but Nicky Wroe twice headed off target and Paul Brayson was denied by a fine block from Wright.
Town should have been two up a minute before the break after some great work from Forrest on the right. He squared the ball for Darryn Stamp who could not make sufficient contact to nudge the ball over the line from close range.
Having conceded goals early in the second half of games this season it was imperative that the Shaymen did not allow City back after the break.
And they were well on top when the game restarted with Stamp almost catching out Henderson for a second time with a speculative 35-yarder that had the keeper scrambling.
Moments later, Daryl Taylor was denied the chance to get on the end of a Stamp cross by a defender and when Toulson picked out Steve Torpey in the area, the winger turned well only to see his shot deflect off a defender and across the face of goal.
The second goal finally arrived in the 52nd minute and it was a great run from Toulson, who beat three defenders down the Halifax right, that set up the opportunity for Forrest, who finished emphatically.
That should have been game over and it surely would have been four minutes later had Town bagged a third. Taylor broke before picking out Forrest who was denied a shooting chance by David McGurk's great defending.
It proved to be something of a turning point in the match.
Two minutes later Stuart Elliott played the ball in for Brayson who squared to Sodje five yards out and he pulled a goal back for the home side.
And three minutes later Brayson crossed from the left, the ball ricocheted off Adam Quinn and Martyn Woolford side footed home from six yards.
Town responded positively with Taylor finding Stamp down the right and his cross was turned behind by Darren Kelly. Taylor was twice denied from the resulting Tom Kearney corner, initially by a defender, then the post.
But City had more in the locker and hit the front with 20 minutes remaining as Sodje surged into the area and pulled the ball back for Meechan who had the simplest of tasks - again from close range - to fire home.
Junior Dadson, on his Town debut, tried his luck from distance and saw the ball hit a defender before bouncing wide. And the youngster was inches away from connecting with a Taylor cross moments later as the Shaymen continued to search for an equaliser.
The final opportunity fell to Forrest, from a Taylor touch, but his shot from the edge of the area did not dip enough, bounced off the top of the bar and to safety.