Two bits of poor defending and a lack of cutting edge in attack combined to send Chesterfield out of the FA Cup at the hands of Grimsby Town.
The League Two Mariners, who were dominated for large parts of the game, were clinical with two chances, but the hosts played a part in both goals.
Martin Allen, who was again the subject of angry chants from the Kop, was left to rue his side's lack of creativity in the final third, where many attacks fizzled out.
Defeat in the second round tie ended Town's 12-game unbeaten run in all competitions.
Six minutes had elapsed before there was a semblance of a chance, Jonathan Smith's header from Jerome Binnom-Williams' free-kick drifting wide.
The Spireites captain then sent a 30-yard plus drive goalward, James McKeown saving well.
Chesterfield were desperately unlucky not to take the lead on 13 minutes, Binnom-Williams sliding it through for Alex Kiwomya, his ball into the middle creating an almight scramble on the goal-line, the Mariners somehow escaping with only a corner conceded.
For the opening 37 minutes Chesterfield were by far the better side, more effective in and out of possession than their visitors and pressing quickly to shut down Grimsby attacks.
Smith's combative presence in the middle made the Spireites difficult to play through, while Lee Shaw and Kiwomya posed problems down either flank.
But against the run of play the League Two side went ahead, a corner from the left was flashed to the back post by Wes Thomas and an unmarked Charles Vernam found the net after the ball landed at his feet.
Chesterfield began the second half brightly, attacking from the off with a number of set-pieces, one of which saw Will Evans go down in the box appealing for a penalty that never came.
Levi Amantchi and Zavon Hines replaced Tom Denton and Jordan Hallam and Amantchi was involved in one of Town's better attacks, running down the right, cutting in and finding Curtis Weston who brought a save from McKeown.
Weston had a second go from 20 yards a minute later and this one was deflected just past the post.
For all their possession and territorial dominance, Chesterfield were left facing a mountain to climb when Haydn Hollis went in for a challenge he didn't need to make and left Town exposed, Harry Clifton racing in behind Brad Barry to beat Callum Burton at the second attempt.
With Chesterfield chasing a goal, the game opened up for the Mariners but despite several late opportunities on the counter attack, they were unable to add to their lead.
Chesterfield: Burton, Barry, Binnom-Williams, Evans, Hollis, Smith, Weston, Hallam (Hines 57), Shaw (Rowley 77), Kiwomya, Denton (Amantchi 56). Not used: Anyon, Nelson, Fortune, Muggleton.
Grimsby: McKeown, Hendrie, Fox, Collins, Davis, Embleton, Pringle (Cooke 86), Hessenthaler, Clifton, Vernam (Rose 73), Thomas (Cardwell 79). Not used: Welsh, Russell, Whitmore, Buckley.
Goalscorers: Vernam 37, Clifton 70
Yellows: Clifton 71
Referee: Michael Salisbury. Assistants: Grant Taylor, Anthony Moore.
Attendance: 4,537 (1,694 away)