Albion made an embarrassing exit from the Carling Cup, snatching defeat from the jaws of victory.
A place in Saturday's second round seemed assured for Mark McGhee's much-changed side after substitute Jake Robinson had put them 2-1 ahead in the 80th minute.
League Two toilers Shrewsbury had other ideas, second half substitute Jay Denny emerging as an unlikely hero. The American-born former Stoke midfielder levelled in the last minute of injury time and clinched a famous victory for the Shrews in the first minute of extra time.
It was the last thing the Seagulls needed following their winless start in the Championship.
They only had themselves to blame for letting Shrewsbury back in when the tie seemed done and dusted but McGhee yet again had cause to rue the refereeing of Trevor Parkes.
The Birmingham official denied Robinson a couple of obvious looking penalties, even booking the young striker on the second occasion for an alleged dive.
Parkes sent off Leon Knight at Luton two seasons ago and he upset McGhee last season with two decisions which led to goals in a defeat at Crewe.
The only consolation for McGhee amid the carnage is that Albion suffered similar humiliation at the same stage of the competition last season at home to Bristol Rovers before embarking on a much better run of form in the League.
McGhee made five changes to the team beaten at Hull on Saturday.
Florent Chaigneau made his debut in goal in place of Wayne Henderson, who Aston Villa did not want cup-tied.
Chaigneau's compatriot Sebastien Carole was given his away debut on the right at the expense of Robinson.
McGhee also rested the experienced trio of Jason Dodd, Charlie Oatway and Richard Carpenter, their spots going to Adam El-Abd, Alexis Nicolas and Mark McCammon.
This allowed for a return to 4-4-2, the most interesting aspect of which was Knight starting in the centre of midfield again and taking over the captaincy in Oatway's absence.
Guy Butters was a more obvious candidate but McGhee spoke to the veteran centre half beforehand and had his backing for the decision to give Knight more responsibility.
Perhaps it was a sense of responsibility which caused Knight to unselfishly lay an early free-kick from 25 yards sideways into the path of Colin Kazim-Richards.
It gave the young striker a chance to get off the mark after his frustrating start to the campaign and he almost took it with a low drive which Joe Hart, Shrewbury's 18-year-old keeper, grabbed at the second attempt.
It took Albion 17 minutes to get their noses in front. Carole, bright and breezy, crossed from the right for McCammon, who stooped to conquer with a firm header from ten yards.
In spite of the breakthrough, the 46 places separating the sides was not immediately apparent.
Shrewsbury, playing more football than you might expect of a team towards the fag end of League Two could have equalised swiftly, Ben Smith shooting wide from 15 yards in the 20th minute as Paul McShane lunged to challenge.
The Seagulls failed to heed the warning, the home side restoring parity in the 22nd minute.
Colin McMenamin's header inside the box was turned back from beyond the far post by Jamie Tolley for the widely travelled Mark Stallard to rifle past Chaigneau.
McCammon's battering ram style posed problems for the Hull defence in the second half on Saturday. Shrewsbury were finding the strapping front man something of a handful as well, his header down to Paul Reid offering the Australian an opportunity to restore Albion's advantage. He so nearly took it with a crisply struck half volley which whistled inches wide.
Albion's agitation at being unable to assert their superiority was emphasised in the closing stages of the first half. They were giving away too many free-kicks and Shrewsbury, growing in confidence, almost took a shock lead with a diving header wide at the near post by McMenamin from Stallard's cross.
It was another disappointing evening for Kazim-Richards. He was replaced seven minutes into the second half by Robinson.
Although Kazim-Richards had McCammon for support on this occasion, he will probably be better suited to operating alongside a target man like injured Argentinian Federico Turienzo.
Robinson almost made an immediate impact after Nicolas charged down an attempted clearance. Robinson appeared to be upended by Hart but referee Parkes waved away appeals for a penalty.
The Seagulls had by now upped the tempo. McCammon failed to connect with a dangerous cross from Carole when a goal beckoned.
It was Carole's last meaningful contribution, the introduction of Albert Jarrett prompting a midfield reshuffle. The substitute took up his customary position on the left flank, Dean Hammond moving inside and Knight out to the right.
Albion finally edged back in front with ten minutes remaining, courtesy of a fine three-man move. Knight fed McCammon for a probing cross which was swept in from close-range by Robinson.
Albion, having substituted the substitute Jarrett with Carpenter in a bid to kill the game, conceded on the stroke of full- time, Denny capitalising on sloppy defending following a throw-in with a low drive which sent the tie into the extra period.
Having kept Shrewsbury's hopes alive, super-sub Denny struck again in the first minute of extra time, slotting in a cross from McMenamin.
Albion's angst intensified with another rejected penalty claim, Robinson picking up a booking supposedly for diving when he was blatantly brought down by Kevin Sharp inside the box.
It was a controversial end for the Seagulls to a sorry evening and they now have to lick their wounds before Saturday's tough trip to Preston.
Score: Shewsbury 3, Albion 2.
ALBION (4-4-2): Chaigneau 6; El-Abd 6, McShane 7, Butters 6, Reid 7; Carole 7, Nicolas 7, Knight 7, Hammond 7; McCammon 8, Kazim-Richards 6. Subs: Hart, Carpenter for Jarrett (withdrawn 88), Martin, Jarrett for Carole (withdrawn 68), Robinson 7 for Kazim-Richards (withdrawn 52).
SHREWSBURY (3-5-2): J Hart; Herd, Hope, Walton; Tolley, Smith, Sorvel, Hogg, Sharp; Stallard, McMenamin. Subs: Lyng, Langmead for Stallard (withdrawn 95), Ashton for Hogg (withdrawn 88), Thompson, Denny for Tolley (withdrawn 46).
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