Mackail-Smith (right) was the hero for Peterborough
Craig Mackail-Smith struck in the dying minutes to earn Peterborough an FA Cup third-round replay against Premier League opponents West Brom.
The League One side caused plenty of problems for the Baggies, with Mackail-Smith hitting the post early on.
But the home side led when Jonas Olsson glanced in a Jonathan Greening corner.
They could not hold onto their lead, however, as Mackail-Smith grabbed his 18th of the season by finishing off Aaron McLean's low cross.
The two side will meet again at London Road on Tuesday 13 January in the replay.
Darren Ferguson's side were well worth a second chance even though Baggies boss Tony Mowbray fielded his strongest available side.
Mackail-Smith was a handful throughout against an Albion side that completed a Carling Cup and FA Cup double over Peterborough last season.
He was denied by the woodwork after 22 minutes when he shook off the challenge of Leon Barnett on the left-hand side of the box and his curling effort beat Scott Carson only to rattle against the crossbar.
Carson was the busier of the two goalkeepers and he went down to parry a low drive from McLean before grabbing the ball at the second attempt.
Chris Brunt had a fierce drive blocked by Craig Morgan but Posh were looking the more menacing and George Boyd would have been disappointed to slice a shot well wide.
The Baggies improved after the break and Luke Moore got behind the Posh defence, but after cutting back inside, his shot was blocked by Joe Lewis who had come off his line.
Lewis then parried a low drive by Greening and then clung on to the follow-up effort from substitute Jay Simpson.
With less than half an hour remaining Olsson broke the deadlock when Greening floated over a left-wing corner and the Baggies defender was first to react to steer a header into the corner.
Mackail-Smith should have equalised five minutes later but sent his header wide with the goal at his mercy from a Boyd cross.
Boyd had the ball in the net but he had strayed offside, however with three minutes left Mackail-Smith forced a replay when he turned home a cross from McLean.
West Brom manager Tony Mowbray:
"We can't have any complaints. They worked extremely hard and played to their strengths.
"There was a different atmosphere in the stadium. The FA Cup probably doesn't reach the intensity of the Premier League and yet you've got to try to make that intensity on the football field.
"I felt they deserved their replay. We will go to London Road again and try to get a positive result like the last couple of times we have been there."
Peterborough manager Darren Ferguson:
"The lads did us proud. We caused them problems. Last season we didn't do ourselves justice against them in the two cup meetings but we did today and put on a very good performance.
"I just thought we had weathered it and they scored from a set-piece - which was disappointing on our part.
"I never felt the game was beyond us and in the end we got our rewards. I thought we caused them problems and Craig has done what he does best, sniffed out a goal for us."
West Brom: Carson, Hoefkens, Olsson, Barnett, Cech, Koren (Simpson 46), Greening, Borja Valero, Brunt, Moore (Dorrans 70), Bednar (Beattie 76).
Subs Not Used: Kiely, Robinson, Kim, Donk.
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