Millwall substitute Liam Feeney scored in second-half stoppage time to knock Southampton out of the FA Cup in a shock result.
Millwall are 19 places and 23 points behind second-placed Southampton in the English Championship, but still managed to come from 2-1 down with just 13 minutes to go to win 3-2.
Kenny Jackett's side played five in midfield and took advantage of the extra man to dominate the early stages of the match and made that count in the 17th minute when Liam Trotter finished off a nice move.
But it was 1-1 at the break after Southampton midfielder Adam Lallana scored in the 35th minute.
Southampton headed into the FA Cup fourth-round replay without a win in five games and the home side went for it in the second half, especially after the introduction of Ricki Lambert from the bench.
Lambert had seemingly booked Southampton a fifth-round tie against Bolton Wanderers when he scored in the 77th minute, but goals to Danny N'Guessan and a wonder strike in injury time from Feeney sent the visitors through instead.
In the other FA Cup fourth-round replay, Lomana Tresor Lua Lua and Ludovic Sylvestre both scored stunners in Blackpool's 3-0 win away to Sheffield Wednesday.
Matt Phillips gave the visitors the lead in just the seventh minute when he collected Lua Lua's blocked shot and placed the ball into the bottom corner from the edge of the box.
Eight minutes later, Lua Lua sent the visiting Blackpool fans crazy with a brilliant goal.
The 31-year-old broke from halfway and sprinted down the left hand side before cutting inside at the edge of the box and curling his shot into the far top corner.
Sylvestre did his best to copy his teammate in the 54th minute.
The French midfielder was released by Phillips' back heel and from a very similar position to Lua Lua and with very similar technique, curled the ball past Wednesday's goalkeeper Stephen Bywater into the top corner.
The win means Blackpool will take on Everton in the fifth round of the competition.