A mix-up between Lynn keeper Chuck Martini and defender Glen Fuff gifted Grantham an equaliser which meant Linnets sunk back into the relegation zone.
Manager Gary Mills blasted the 'diabolical' defending after much good work by the home back four had denied a persistent spell of Grantham second-half pressure.
After a Dave Puttnam special, Martini conceded his first goal in over seven hours of football and the draw moves Lynn down one place to third from bottom.
"It was a diabolical equaliser," fumed Mills. "It was the keeper and Fuff, neither of them made a decision.
"Grantham played better than us although as much as they had the ball they weren't hurting us. We know we haven't played well as a team and individuals.
"It's disappointing when you're 1-0 up at half-time at home and I feel we were comfortable."
Nevertheless it stretches Lynn's unbeaten 1999 run to seven games.
"Chuck had a couple of saves in the first half but none in the second," added Mills. "And Dave stuck the goal in well."
In a competitive 'grudge' match Mills and Paul Ramsey passed fitness tests so four of Lynn's team; with Craig Clark and Lee Wilson, were facing their former club.
Former manager of the Gingerbreads Mills was given no special favours, needing a couple of minutes' treatment after being clattered out of touch by opposite number John Andrews.
The atmosphere and match soon buzzed. As early as the first minute Martini dropped the ball and, outside his six-yard box, had to save from Darren Bowman. He then did well to deny Mark Quayle's effort while Fuff blocked the follow-up.
Lynn took a sensational lead with their only effort of the half. Mark Turner headed the ball into the Gingerbreads' box and it fell to Dave Puttnam Despite having his back to goal he rifled in an outrageous overhead kick which looped over former Scotland under 21 keeper Gary Germaine.
This sparked a brief., bout of fighting among a few supporters behind Martini's goal and it was later confirmed that eight away fans had been ejected from the ground.
Martini, just called into the Moroccan B team, must have been feeling the pressure as he missed his punched clearance, but Quayle put wide a gilt-edged open goal chance.
Just before half-time, from a Puttnam corner Fuff was presented with the ball in acres of space but he miskicked.
Early, in the second half Turner headed on Clark's quick free-kick and Wilson's shot was well saved. Then Turner directed a weak header straight at the keeper.
Grantham reacted with a lengthy period of possession and pressure, around the Lynn box. But the four man Linnets defence coped well with Tony Spearing outstanding.
However a moment of confusion: between Martini and Fuff, with neither taking charge in a back-pass situation, resulted in on-loan Peterborough forward Chris Cleaver nipping in.
Fuff was lucky not to concede a penalty as he barged Cleaver but Quayle did well in scoring from an acute angle, a replay of the last goal Lynn conceded at Salisbury five matches ago.
Wilson had two late opportunities. He shimmied past three defenders and produced a diving save from Germaine.
Then in injury time, fans in the main' stand thought Wilson had grabbed the winner as he hit the outside netting at the far post.
Lynn: Martini, Hoyle, Clark, Fuff, Spearing, Mills, Turner, Ramsey, Allen, Wilson, Puttnam (McGinty 86). Subs (not used): Palmer, Hilton
Grantham: Germaine, Dakin (Stahlberger 51), Goodwin, Nwadike, Spped, Andrews, Bowman, Quatle, Cleaver, Gilbert, Glasser. Sub not used: Pell, Kirkham.