Trailing 1-0 to a ten-man Shrewsbury Town they grabbed an equaliser two minutes into added time.
Leading scorer Danny North stuck out a foot to get on the end of a shot from Danny Boshell to turn the ball into the top corner.
Amazingly, North could have won it seconds later for the Mariners with his header arching for the top corner, only to be denied by a flying save from the impressive Glyn Garner.
But defeat would have been a travesty for impressive the Shrews, who were the better side for most of the game, shrugging off the loss of central defender Colin Murdock on 55 minutes for bringing down North.
Shrewsbury re-organised, dusted themselves down and took the game to the home side.
Impressive striker Guy Madjo, a £20,000 buy from Crawley Town, got on the end of a blocked shot to give the visitors the lead.
And then as Grimsby went with three men up front, they were carved open and the visitors had chances to have put the issue beyond doubt.
But with the fourth official signalling three minutes added time, there must have been some tired legs in the visitors' line-up.
There was an air of exasperation from Grimsby as they pressed forward and the corner led to the equalising goal with two of the three minutes having been played.
In a poor first half, Shrewsbury had the better of things against a lacklustre Grimsby and skipper Ben Davies ought to have put the Shrews ahead just before half-time.
Played clear, he measured his shot which was blocked by Grimsby keeper Phil Barnes, who was as impressive as his opposite number.