Villa's promising start to the season came to an abrupt halt when they stumbled out of the Capital One Cup at the first hurdle.
Just as everyone at Villa Park was looking to extra-time, sub Romain Vincelot headed an 87th-minute winner which sent Leyton Orient into the third round.
It was only the second attempt on target by the League One side, although Shay Given had earlier denied the visitors the lead with a magnificent save.
Villa, showing seven changes from Saturday's goalless draw against Newcastle, dominated for long periods, with teenager Jack Grealish producing an eye-catching performance in his first full game.
But Paul Lambert's men were unable to find a cutting edge and eventually paid a heavy price.
Leandro Bacuna, Carlos Sanchez and Alan Hutton all delivered early crosses as Villa found space down the flanks.
And although none was productive, Bacuna's was preceded by a delightful pivot which took him away from his marker and earned rich applause from home supporters.
Bacuna then switched wings, cutting inside from the left for a well-struck shot which hit Mathieu Baudry's forehead, before Nathan Baker headed wide from Ashley Westwood's 23rd-minute corner.
The first real threat of a breakthrough came on 28 minutes, debut boy Joe Cole accepting Westwood's short pass and releasing Darren Bent through the right channel with a superb ball.
The club's record signing looked set to open the scoring but his low angled shot clipped keeper Gary Woods' outstretched leg and deflected for a corner.
That move gave Villa extra impetus and a minute later Grealish produced some clever footwork before sending a square pass to Westwood, whose 25-yard right-foot drive flashed not far off target.
Despite the home side almost constant pressure in the opening half hour, the visitors might have taken a surprise lead in the 36th minute, Elliott Omozusi's right-wing centre almost appearing to take Dave Mooney by surprise as the Orient skipper headed well wide from a good position.
As Villa resumed the initiative, Bent just failed to connect with Hutton's dangerous cross from the right, although Mooney suffered the same fate just after the interval when he stretched but just failed to get his head on to Jay Simpson's delivery.
The home side's first effort of the second half saw Sanchez cut inside from the left to send an angled right-footer beyond the far upright.
Then Bacuna's shot was charged down after some excellent approach work by Westwood and Bent.
Grealish's silky skills were causing endless problems for the Londoners and in the 61st minute the young midfielder unleashed a stinging drive which went behind off a defender.
When Westwood sent over another fine flag kick from the right, Baker climbed highest for a header that only just cleared the bar.
But Orient went even closer on 68 minutes, Given brilliantly tipping the ball against the inside of the post when Baudry met Cox's corner with a firm downward header which looked a certain goal.
It was the first time Given had been required to make a save and a minute later the visitors had the ball in the net, although Simpson was clearly offside as he tapped home from close range.
In the 70th minute, Grealish wasn't far away from his first senior Villa goal, his snap shot going only narrowly wide after some impressive build-up play from Hutton.
It was the turn of Philippe Senderos to impress in the 83rd minute, the Swiss defender sending Bent racing clear with a precision pass, only for the striker to fire wide.
But after Baker had sent a glancing header off target, Vincelot pounced with his far post header following a cross from Bradley Pritchard.
VILLA: Given; Hutton, Senderos, Baker, Richardson; Westwood, Sanchez; Bacuna, Cole (Weimann 62), Grealish; Bent. Unused subs: Guzan, Clark, Agbonlahor, Delph, Herd, N'Zogbia.
ORIENT: Woods; Omozusi, Baudry, Lowry, Sawyer (Cuthbert 62); Pritchard, James, Bartley (Vincelot 83), Mooney (Dagnall 66), Cox; Simpson. Unused subs: Legzdins, Clarke, Lee, Henderson.