Sky Bet League Two leaders Exeter fell to their first league defeat of the season as they were humbled 3-1 by Grimsby at St James Park but managed to hold onto top spot on goal difference.
Exeter started brightly, but were hit on the counter when Grimsby opened the scoring after 23 minutes. Jake Hessenthaler's corner found Ethan Robson completely unmarked and he headed powerfully past Lewis Ward.
Grimsby looked threatening on the counter attack, but could not break the Mariners down. Nicky Law saw a shot blocked just before half-time and Ryan Bowman failed to connect with Jack Sparkes' cross when it looked easier to score.
Sweeney then truck the crossbar for Exeter before Law's follow-up was cleared off the line and onto the bar and Grimsby hit Exeter again on the counter on the hour mark.
Dean Moxey could have no complaints as he brought down Max Wright and Moses Ogbu fired the resulting penalty past Lewis Ward to double Grimsby's score.
The Grecians were caught out again after 67 minutes as Ogbu teed up Robson to smash in his second of the game as Grimsby took advantage of the gaping holes in Exeter's backline.
The home side did bag a consolation in stoppage time as Bowman swept in a Randell Williams free-kick, but it was too little too late for Matt Taylor's men.