Chesterfield looked set for the three points after a dream start with two goals from Eoin Doyle in the fourth and 22nd minutes and one from Gary McSheffrey after 10 minutes gave the Spireites the advantage.
Morecambe were given a brief glimmer of hope when Kevin Ellison pulled one back 40 seconds into the second half.
When home skipper Mark Hughes scored a second on the hour-mark the game became even more interesting.
With Morecambe finding new belief Jack Sampson poked home Ryan Williams's far post free-kick to bring the sides level and the comeback was complete when with six minutes to go Padraig Amond headed Sampson's left-wing cross past Tommy Lee to seal a remarkable second-half show from Bentley's side.
Chesterfield had chances of their own to score a fourth with Jay O'Shea firing wide from a good opening and Barry Roche saving well to deny Doyle his hat-trick as Cook's side slipped to a second successive defeat.
Cook said: "We were fantastic in the first half and played exactly like league leaders should do.
"We were bright in possession and we created lots of chances.
"But we didn't turn up in the second half it is as simple as that and if you are not at it against any team in this league you will get hurt.
"You have to give Morecambe credit for the way they fought their way back into the game, but we should have handled things a lot better than we did.
"We have a side full of experience, but those who you expect to lead didn't do that and we were made to pay.
"When you are three-up you just need to kill the game off and we failed miserably.
"We had conceded four goals in nine games before this game and four in one half and that says everything about the game."