JOE Mattock's first-ever Albion goal three minutes from time kept the club's FA Cup dreams alive by taking this fifth-round thriller to a Hawthorns replay on Wednesday week.
Reading sub Simon Church's 73rd-minute strike appeared to have sentenced the Baggies to a second competition exit at the Madejski Stadium inside four years.
But Mattock had the final say of an end-to-end tie which saw both teams finish with ten men.
Albion got off to the worst start imaginable when Royals winger Jimmy Kebe broke the deadlock after just 9.5 seconds - believed to be the fastest-ever goal in the FA Cup's 'proper' rounds.
But Robert Koren clawed the Baggies level in the 18th minute with his second goal of the season.
The tie took two dramatic twists when both Coca-Cola Championship teams had players sent off in the opening 15 minutes of the second half.
Reading striker Shane Long was the first to go for a late lunge on Abdoulaye Meite in the 48th minute before half-time sub Youssouf Mulumbu followed him off with two quickfire yellow cards.
There was late drama prior to the game when Andy Slory was denied his full Albion debut.
The Dutch winger was set to replace the cup-tied Frank Nouble on the left wing but injured his heel in the warm-up.
Originally a substitute, Koren was promoted to the starting XI, with 17-year-old Kayleden Brown drafted onto the bench where he was joined by fellow Academy product Sam Mantom.
Roberto Di Matteo made three changes in total with Gianni Zuiverloon, Meite and Simon Cox coming in for benched duo Gabriel Tamas and Mulumbu, plus Nouble.
The drama off the field was matched on it as the Royals took the lead with what TV crews timed as a goal after only 9.5 seconds.
Albion kicked off but after the ball was played back to Zuiverloon, the right-back was dispossessed by the recalled Kebe who drove into the box and slotted past Scott Carson.
The Baggies regrouped and threatened in the fifth minute when Roman Bednar flicked on Chris Brunt's right-wing corner at the near-post and Cox headed just over.
Five minutes later, the Baggies had a lucky escape when a Reading free-kick saw Brian Howard loft the ball to the far post. Kebe headed wastefully into the side-netting when he should at least have hit the target.
The visitors finally clicked into gear in the 18th minute when Graham Dorrans burst forward and found Brunt on the right.
The Northern Ireland international swung the ball to the far post where Koren had the simplest of tap-ins to level.
A minute later, Bednar came within inches of snatching the lead but just could not stretch far enough to make contact with Cox's ball into the six-yard box from the right.
Long managed to spin Jonas Olsson in the 26th minute and drill in a low shot from the edge of the box but Carson was equal to it, diving low to his left to grab at the second attempt.
Five minutes later, Bednar let fly with a swerving 30-yard strike which Adam Federici just managed to beat out to safety.
Koren was unfortunate not to score his second of the game in the 33rd minute. The ball fell to the Slovenia skipper on the edge of the box from Matt Mills' header and he crashed a volley off the left-hand post.
Brunt could then have headed Albion in front in the 40th minute, only to sky his close-range effort from Dorrans' left-wing corner.
Albion were well on top now and, a minute later, Bednar's near-post flick from Koren's right-wing cross was clutched by the flying Federici.
The first half finished with Gonzalo Jara needing treatment following a 50/50 challenge and he did not remerge after the break, with Mulumbu replacing him
Within three minutes of the restart, the hosts were reduced to ten men when Long stamped on Meite's ankle. He was swiftly shown a straight red card by referee Chris Foy.
Brian McDermott then made a change as Howard made way for Church.
Meite was also unable to continue and Tamas replaced him in the 52nd minute.
Dorrans powered into the box in the 56th minute and the ball rolled for Bednar, whose stabbed effort ran inches past the left-hand post, with Federici beaten.
Four minutes later, the Baggies were also reduced to ten men when Mulumbu fouled Jem Karacan and promptly received his second yellow card of the game.
The hosts made another change in the 63rd minute as Gzregorz Rasiak came on for Kebe.
The ball fell kindly to Jobi McAnuff on the edge of the area three minutes later and he curled his effort just past the right-hand angle of post and bar.
Mattock tried his luck at the other end in the 67th minute and saw his long-range effort fly just past the right-hand post.
But Albion fell behind again in the 73rd minute when sub Church beat Albion's offside trap to race onto Rasiak's throughball.
The Royals striker's initial effort was brilliantly blocked by Carson but the rebound fell kindly for him and he slotted home.
But Albion were not dead and buried yet and Cox ran onto Tamas' long ball in the 77th minute and cut inside Andy Griffin before seeing Federici save his low effort at the foot of his near post.
Play switched to the other end where Carson first beat away a Church drive from the edge of the area before Rasiak fired McAnuff's back-post corner over.
There was to be one final dramatic twist - in Albion's favour.
After a forceful run by Dorrans in the 87th minute, Cox played the ball out to Mattock on the edge of the box and he fired a terrific drive across Federici into the far bottom corner to force a replay.
READING (4-5-1): Federici; Griffin, Ingimarsson, Mills, Bertrand; McAnuff, Howard (Church 49), Karacan (Tabb 84), Gunnarson, Kebe (Rasiak 63); Long. Subs not used: Hamer (gk), Henry, Robson-Kanu, Pearce.
ALBION (4-5-1): Carson; Zuiverloon, Meite (Tamas 52), Olsson, Mattock; Brunt (Moore 73), Cox, Jara (Mulumbu HT), Dorrans, Koren; Bednar. Subs not used: Kiely (gk), Brown, Wood, Mantom.
GOALS: READING - Kebe (1), Church (73). ALBION - Koren (18), Mattock (87).
BOOKINGS: READING - Howard (foul 44). ALBION - Mulumbu (foul 47), Tamas (foul 65).
SENDING-OFFS: READING - Long (serious foul play 48). ALBION - Mulumbu (second bookable offence 60).
REFEREE: C Foy (Merseyside).
ATTENDANCE: 18,008 (Albion fans 4,416).
ALBION STARMAN: Graham Dorrans.