Charlton midfielder Bradley Pritchard was at a loss to explain the 2-1 defeat by Sheffield Wednesday.
The Addicks were 1-0 up with six minutes remaining and looking well on their way to a fourth straight league win.
But Reda Johnson's header levelled things up before new Owls striker Leroy Lita grabbed a late winner for the visitors.
"We were 1-0 up and doing well, keeping our shape, and I honestly couldn't see them scoring," said Pritchard.
"Maybe we got tired, a loss of concentration here and there and they capitalised. They play a certain way, they keep hammering at the door. It was really, really disappointing."
Pritchard played a key role in Charlton's opener, opting not to shoot from Chris Solly's cross but instead teeing up captain Johnnie Jackson for his seventh goal of the season.
"I think the way my shooting has been recently, I was probably playing the percentage game there," he admitted..
"Johnnie Jackson is in a good run of form and he always puts them away, so if he was in a better position I was just happy to be involved."
The League One side secured one of the results of the fourth round, progressing to the last 16 and securing a home tie against Barnsley after they won 4-2 at Loftus Road on Saturday.
Former Leeds, Manchester United and Newcastle man Smith was the most experienced player in the Dons side and one of few with any Premier League experience.
The 32-year-old, a former England international, has also tasted Champions League football and was part of the Leeds side that beat both Milan and drew with Barcelona during the 2000/01 season.
Asked if it was silly to compare the Dons' win to such results, he said: "Not when you are coming to the end of your career. You remember them all.
"This one will be remembered as fondly as some of those results. Obviously it was not as spectacular as who you were playing against - they were all European giants.
"But it is a Premier League team who have spent a lot of money and we came and played good football. It is not as though we were backs to the wall and a 1-0 victory. It proved how good we can be.
"But the main focus has to be the league. The main thing has to be picking ourselves up for the game on Tuesday against Yeovil. That's reality. That's the one you have to be motivated for.
"These games are easy to get up for. These ones you are free to go out and express yourself. But the bread and butter is getting out of League One.
"That is proving difficult because there are so many good teams in there. We proved we are one of them."
Forrester was denied securing the biggest giant-killing act in what is becoming a season of sensational cup upsets after Fernando Torres' cute finish snatched a late 2-2 draw in the fourth-round tie at Griffin Park.
European champions Chelsea were simply bullied by their opponents throughout the lunchtime contest, with Forrester's penalty looking like the winner before Torres struck seven minutes from time.
Marcello Trotta had fired the hosts in front moments before half-time but Oscar equalised for Rafael Benitez's Blues with a delightful strike.
Brentford's hopes of victory in the replay at Stamford Bridge in three weeks' time have already been written off, but man of the match Forrester insists they could still spring a surprise.
"Yes, but we will still be the underdogs which we will take," he said. "We will go there and enjoy. We will have a different game plan again.
"For the replay, we can say we have done it once, we got a draw, we maybe should have got more. We won't be scared, we weren't scared here. We came out to let our football do the talking.
"To come out 2-2 against the champions of Europe and be half disappointed shows how far we have come. We have gone ahead, got seven or eight minutes left and we let a goal in.
"Man for man, they are better but, collectively, we can show we have got the talent and determination.
"We all know what we have got in the changing room. We have got some good players even in the third tier of English football. We didn't let the occasion overawe us."
The 27-year-old, who has been capped twice for Scotland, played for both Dundee and Dundee United, and moves to Easter Road following a brief spell with the Tangerines.
Robertson told Hibs' official website: "This is a great move for me and I'm really excited about what we can achieve at Hibs.
"The club has made a decent start to the season and we are in a strong position in order to be able to kick on and progress even further.
"There were other options on the table, but as soon as I spoke to Pat Fenlon I was convinced that Hibs were the right club for me."
Hibs boss Fenlon says he was extremely pleased to be able to add Robertson to his playing staff.
He said: "Scott will be a valuable addition to our squad - he is a skilful box-to-box midfielder, who is strong in the air and can provide goals.
"He is a player, who has been on our radar for a while, so we were pleased to be able to get the deal over the line.
"Again, Scott has signed until the summer of 2015 which brings continuity and we believe he is the type of player our fans will enjoy watching perform."
