It was a game of two halves at the Proact Stadium, as Chesterfield exited the FA Cup at the Second Round stage.
The Blues were unbeaten in 12 in all competitions heading into the Sunday afternoon tie, but goals from Charles Vernam and Harry Clifton handed visitors Grimsby Town a place in the Third Round.
Anyone would be forgiven for confusing which side was in the Football League and which wasn't in the first half, as the Spireites created chance after chance without reward.
Grimsby will feel very fortunate not to have fallen behind, and took the lead with their first shot of the game. Poor defending from a corner saw Vernam with enough space take a touch before sliding the ball into the net.
Chesterfield were visibly shaken by the opener, and failed to regain their impressive shape and play of the first half-hour, and entered the break a goal down but far from out.
The second half, however, was more reminiscent of the end of the first half than the beginning, as whilst Martin Allen's men struggled to get a grip on the game, it was the Mariners' turn to pile on the pressure, looking likely to score the second goal of the afternoon.
They eventually did through Harry Clifton, as a breakaway saw the visitors two-on-one and the helpless Callum Burton in the Town goal did well to save the first effort with due care and attention not to knock it into the path of supporting Davis, but Clifton bundled in the loose ball.
Chesterfield's unbeaten run comes to an end, but they'll resume their search for a first league win since August against Salford on Saturday.