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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.
Substitute Stephen O'Flynn's final minute diving header salvaged an unlikely point for nine-man Crusaders against Coleraine at Ballycastle Road.