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Paul Heatley was sent-off in the 41st minute following an off-the-ball incident with Lyndon Kane.
James McLaughlin turned the ball home from six yards midway through the second half after Sean O'Neill had parried Ruairi Harkin's initial shot.
Jordan Forsythe was dismissed for a crude two-footed challenge on Kane.
The precious point keeps Stephen Baxter's champions within four points of Premiership pacesetters Linfield.
Crusaders went into the game on the back of five straight league wins while the Bannsiders' record of just two wins in their previous 21 matches against their visitors suggested they faced an uphill task.
Rodney Brown directed a header straight at O'Neill and then Forsythe headed wide and saw a close-range effort cleared off the line by Steven Douglas.
Heatley had a low shot parried by Coleraine stopper Michael Doherty, while Jamie McGonigle raced clear and found himself one-on-one with O'Neill, who blocked the Coleraine player's shot with his legs.
Heatley was shown the red card by referee Keith Kennedy and when McLaughlin broke the deadlock, Oran Kearney's side looked to be in control.
Forsythe's 71st-minute sending-off appeared to seal the Crues' fate but O'Flynn's late intervention after a Craig McClean free-kick earned a last-gasp draw.

Shyamal Kanti Bhakta, who is a Hindu, was made to hold his ears while performing squats, before being beaten.
Many Bangladeshis were angered by his treatment, posting photos of themselves holding their ears in solidarity.
The government said the sacking was illegal and dismissed the school board. Mr Bhakta denies insulting Islam.
"The head teacher was a victim of injustice. This was a heinous act," Education Minister Nurul Islam Nahid said.
But he made no mention of local MP AKM Selim Osman who was present at the teacher's ritual humiliation in Narayanganj District near Dhaka last Friday.
The MP has refused to apologise for what happened, the Daily Star reports.
In footage of Friday's incident, a crowd can be heard cheering as Mr Bhakta was made to squat repeatedly and folds his hands - an action associated with shame and apology and usually reserved for children.
Video and photographs showing Mr Bhakta's treatment soon went viral on social media.
Mr Bhakta has told the BBC his treatment was the result of personal grudges against him.
It comes in the wake of a series of attacks on secular writers and bloggers, professors and members of religious minorities in the country.
Substitute Stephen O'Flynn's final minute diving header salvaged an unlikely point for nine-man Crusaders against Coleraine at Ballycastle Road.
[NEXT_CONCEPT]
The Bangladeshi government has reinstated a headmaster who was sacked after being publicly humiliated over allegations that he insulted Islam.