Nott'm Forest 2-3 Oldham
League One strugglers Oldham Athletic produced a stunning second-half fightback to send Championship side Nottingham Forest out of the FA Cup.
Matt Smith turned the ball into his own net to gift Forest a first-half lead.
But Robbie Simpson followed up a brilliant 25-yard strike with a deft header to make it 2-1 to the Latics.
Jose Baxter's stunning free-kick, after Danny Collins was sent off, secured a sensational victory despite Billy Sharp's late strike.
Oldham, 19th in League One and just five points above the relegation zone, fully deserved their incredible win, which puts them into the fourth round for the first time in five years and leaves new Forest manager Alex McLeish still seeking a first win after three games in charge.
And it was not as if the Scot had selected a weak side, no doubt mindful of the need to win over Forest fans still angered and confused by Sean O'Driscoll's sacking.
Oldham, helped by an injury to Reds midfielder Andy Reid just before the break, simply blew away their hosts and made a mockery of a miserable run which has seen them lose four of their last five games.
It had started well enough for Forest who had failed to score in their last five games against the Latics. That record was soon forgotten as they took the lead after 13 minutes.
The visitors had defended well in the early exchanges but they trailed when Smith, under pressure from Simon Cox following a dangerous Reid free-kick, saw the ball rebound off him past goalkeeper Dean Bouzanis from six yards.
Forest began to dominate possession and territory and although they struggled to create many clear chances before the interval, the Championship side's superior attacking quality meant they always looked dangerous.
Dexter Blackstock could not find a finish after going through on goal and being forced wide by Bouzanis, and Radoslaw Majewski was only just wide with a fizzing long-range strike.
Oldham's only noteworthy opportunity saw Baxter curl a fine free-kick a fraction wide.
But how things changed after the break. Oldham, previously so cautious, piled men forward and overran their hosts - much to the delight of a noisy away support of just under 3000.
They controlled the midfield, Smith posed countless problems up front and the team played with a confidence and intensity that Forest could not match.
Simpson's 25-yard strike levelled the scores and the same player looped a brilliant header over goalkeeper Lee Camp from Dean Furman's perfect far-post cross to make it 2-1.
Forest's capitulation was complete within moments when Collins was sent off for pulling down Smith as the striker ran through on goal and Baxter's fine free-kick easily beat Camp to increase the lead to two goals.
An increasingly hostile home crowd watched on as the Reds tried to salvage something but Oldham were rarely troubled. All Forest could muster was a hooked strike from Sharp into the far corner deep into stoppage time.
Nottingham Forest manager Alex McLeish:
"We were woeful in the second half. We were sloppy in everything we did and paid the penalty for it.
"We warned the players at half-time that they had to be professional in the second half but we had to be a lot better than that.
"We gave the ball away cheaply, didn't close them down quickly enough and it was all very half-hearted. Their big lad up front [Smith] caused mayhem in the second half and we didn't deal with it. Our centre-halves had to be ugly and rugged to deal with the threat.
"I was pretty pleased with the first-half performance but we looked a little bit frightened after the break. We allowed them to press us and it unnerved us. Why the players allowed that to happen is a mystery to me."