Gunnarsson celebrates his spectacular 25-yard equaliser
Christopher Samba netted deep into injury-time to spare Blackburn's blushes and deny Coventry City a place in the FA Cup quarter-finals.
The Sky Blues trailed to Roque Santa Cruz's fine strike inside 90 seconds.
But Aron Gunnarsson's superb long-range shot drew the visitors level and Michael Doyle's deflected shot looped in to put them 2-1 up.
Rovers piled forward late on and Samba scored on the rebound after Andy Marshall spilled Keith Treacy's shot.
The home side could even have won the match with seconds to go but Coventry eventually cleared their lines after another goalmouth melee.
The visitors will be devastated they must face Rovers again in a replay on Tuesday week (February 24).
But a possible FA Cup humbling had not looked on the cards when they found themselves trailing 85 seconds in to Santa Cruz's shot.
The Paraguayan finished coolly into the corner from an acute angle following Samba's flick-on - the Manchester City target's fifth goal of an injury-ravaged season.
A much-changed Sky Blues were looking a little disjointed and some way off the side that beat Championship leaders Wolves last week.
Rovers were dominating possession and had several chances to extend their lead.
Jason Roberts crashed a glorious half-volley wide and Santa Cruz had two decent opportunities to add to his early strike.
Santa Cruz is all smiles after putting Rovers ahead inside 90 seconds
But he sent his first shot into the side netting and then fizzed a 25-yard strike just wide of the post.
Carlos Villanueva also shot wide for the home side in the first quarter of the match.
But after their nervy start, the visitors did have a couple of decent chances of their own and Gunnarsson should have drawn them level only to head wastefully wide from six yards following Daniel Fox's free-kick.
Jordan Henderson also went close with a shot across the face of goal that beat both Paul Robinson and the far post.
Coventry, who thumped Rovers 4-1 in the third round last season, upped the tempo after the break and drew level on 61 minutes following a spell of sustained pressure.
Gunnarsson took a touch on the edge of the box and crashed in a wonderful right-foot shot into the top corner from 25 yards.
The momentum was with Chris Coleman's men and they grabbed a second goal with less than a quarter of an hour to go.
Doyle ran on to Freddy Eastwood's pass and his deflected effort looped up and in to send the away fans into raptures.
It was a lead they held until two minutes into stoppage time when Samba knocked the ball home.
Sky Blues keeper Andy Marshall, deputising for Kieran Westwood who felt unwell in the warm-up, fumbled Treacy's shot to allow Samba to set up a replay and put both teams into Sunday evening's draw.
Blackburn boss Sam Allardyce:
"I have got no problems with the result. I just have a bit of a problem that we cannot see teams off when we have an opportunity.
"We have started to ship too many goals at Ewood and that is becoming an Achilles heel.
"We lost our fluency but we then had a very good goal (by Gael Givet) chalked off and a free-kick given instead which I cannot understand.
Coventry manager Chris Coleman on keeper Andy Marshall who was a late replacement for Keiren Westwood:
"He has not played for a while but he came in and did a fine job.
"It was difficult for him, the last goal. It was a crowd of bodies and he saw it late. There was nothing he could do.
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