Reading booked an FA Cup quarter-final clash against Aston Villa after staging a remarkable comeback to see off a West Brom side who were themselves within a minute of a meeting their midlands neighbours.
Gylfi Sigurdsson proved to be the match-winner in the fifth minute of extra time, just moments after Brian Howard's strike in the third of four stoppage-time minutes at the end of normal time had brought a dramatic end to the fifth-round replay.
It proved to be an historic night for Reading, with their first win at The Hawthorns since 1995 securing their place in the last eight for the first time since 1927.
But it could have been so different for a shell-shocked Albion side who looked destined for their second quarter-final place in three seasons.
The most relieved man in the Black Country must surely have been Royals goalkeeper Adam Federici, whose calamitous 47th-minute gaff in letting Robert Koren's shot squirm horribly through his body looked to have proved the difference.
Koren had earlier handed the Baggies a sixth-minute lead in the fifth-round replay before Jimmy Kebe levelled almost immediately - both players also scored the first two goals in the initial 2-2 draw between the two sides at the Madejski Stadium.
The final whistle was greeted by a mass melee involving players, stewards and officials, with Reading's Jobi McAnuff shown a red card.
Slovenia captain Koren had already twice gone close when he opened the scoring after just six minutes. Roman Bednar received the ball 25 yards out and his low drive was parried by Federici, with Ryan Bertrand clearing Luke Moore's rebound off the line.
But the Royals full-back could only send the ball straight into the path of Koren who finally found the net at the third time of asking.
Albion's lead did not last long, though, as Kebe, who scored after nine seconds in the first meeting, side-footed home from six yards after Scott Carson had failed to keep hold of Jay Tabb's low shot across the face of goal in a rip-roaring start to the tie.
But the frenetic opening 15 minutes was as good as it got in the first half, with no real goalmouth action of note in the remaining half hour.
Yet the 13,985 inside The Hawthorns were treated to a second-half opening similar to the first although this time Albion took just two minutes to find the net - and they did so in hugely fortuitous fashion.
Youssouf Mulumbu found Koren and he turned on the edge of the area before firing a low shot that went straight through Federici's grasp and into the back of the net.
Reading, however, almost repeated their first-half feat but Sigurdsson's dipping long-range effort came back off the crossbar.
The Iceland Under-21 international was presented with a second opportunity to level the tie in the 69th minute after Gabriel Tamas's mistake but he could only fire straight at Carson.
An ordinary final 20 minutes followed with West Brom seemingly destined for a place in the last eight, that was until Howard silenced the home crowd in the third minute of stoppage time.
And there was worse to come for Albion in extra time as Sigurdsson settled matters with a sublime curling effort from 25 yards - ensuring West Brom were added to Reading's list of scalps in this season's competition, with the Royals having already seen off Liverpool and Burnley.
Brian McDermott insists Reading will not fear FA Cup quarter-final opponents Aston Villa after securing a place in the last eight for the first time since 1927 with a memorable win over West Brom in a pulsating fifth-round replay at The Hawthorns.
Most of the post-match talk surrounded a mass melee after the final whistle involving players, officials and stewards - with Royals winger Jobi McAnuff seeing red for his involvement in an incident with Albion midfielder Youssouf Mulumbu.
McDermott felt the situation was "unnecessary'' and criticised stewards for "definitely over-reacting'', while promising to review television footage of the incident before considering an appeal against McAnuff's dismissal. But predictably, he was more comfortable talking about the Berkshire club's historic achievement.
"For me the key is the football, we're in the last eight and I'm proud of all the players,'' said McDermott. "It was a great night, a fantastic cup tie. You talk about the FA Cup losing its shine but you saw tonight what that meant to two sets of players and supporters.
"It was a fantastic night for everybody in a Reading shirt and everybody connected with the club. It's put us back on the map a little bit and I'm pleased about that. We're one step away from Wembley.
He continued: "To share a touchline with Martin O'Neill is great for me personally. To play Villa, fantastic. I don't really have any fear about facing anybody, full stop. I just enjoy the occasion.''
Albion head coach Roberto Di Matteo did not want to comment on the incident following the final whistle, insisting the Football Association can deal with it. But the Italian could not hide his disappointment at bowing out of the competition.
"I'm destroyed,'' he said. "We were in the driving seat and it wasn't a dangerous situation (Reading's
late equaliser). We just needed to clear the ball. We put a lot of effort in and we worked very hard to win the game, then we come away with nothing.
"We just didn't defend the second and third goals, we gave them gifts to win the game. We threw it away.''
Albion must now turn their focus back to securing promotion from the Championship, and Di Matteo added: "We have to pick ourselves up to get going again on Saturday.''