Forbes levelled with a superb bicycle kick from Robert Atkinson's long throw with just 12 minutes to go to net his first goal for the club since joining on-loan from Millwall.
But it was Barnet who looked the more likely to win the game having thrown away the lead twice after storming into a two-goal lead in the first 20 minutes.
They took the lead after 18 minutes when skipper Neal Bishop fed on-loan Plymouth man Yannick Bolasie and the winger marked his full debut with a superb low strike from 20 yards.
And two minutes later the hosts doubled their advantage when Albert Adomah's cross was headed home by evergreen ex-Chelsea striker Paul Furlong for his third Barnet goal.
But Grimsby came roaring back into the game and pulled a goal back on 25 minutes when skipper Ryan Bennett bundled the ball over the line from Dean Sinclair's corner.
Then on the stroke of half-time the Mariners were level as Sinclair's cross found Stuart Elliott and the left winger nodded home the equaliser.
Barnet looked to start the second period as they did the first and should have put the game beyond the visitors in the first ten minutes of the half.
First debutant Mark Hughes fired just wide when clean through and then John O'Flynn missed an even better chance a minute later when he could only fire straight at keeper Phil Barnes when unmarked in the box.
O'Flynn made amends when he gave Barnet the lead again from the penalty spot on the hour after Adomah was fouled by Joe Widdowson.
But Barnet's joy was short-lived as Forbes connected with Atkinson's throw in style to keep Grimsby clear of trouble at the bottom of League Two.