The England goalkeeper's gaffe gifted Gabby Agbonlahor the opener in the 1-1 draw at Villa Park before Nathaniel Clyne rescued a point for the disappointing Saints.
It was just the sixth goal second-placed Southampton have conceded in the league and meant they are six points behind leaders Chelsea.
But Koeman insisted Forster's first-half mistake - when he came and missed Ciaran Clark's long clearance - is a one-off.
He said: "He is a great goalkeeper and that doesn't change after this mistake.
"It doesn't usually go against us and it was a misunderstanding
He came out, stopped and if you stop you are too late.
"He is clever enough to understand that kind of mistake.
"But we deserved our goal and we had the patience in the second half
We knew they would defend and keep their organisation.
"It was late but well deserved
We played more football than they did
Every game in the Premier League is difficult and they were missing some key players and in a difficult situation and maybe that's more dangerous than normal.
"After being 1-0 down it was more difficult but we got a point and for confidence it was important."
The Saints, who have lost once in their last 14 games, host Manchester City on Sunday and Koeman insisted they will still challenge the top four.
"Chelsea are the team to beat and it's difficult because they have qualities in defending and attacking," he said.
"It's up to the rest, we'll try to keep our position and way of playing because we believe it'll bring points but after 38 games I don't believe we will be second in the table, maybe third."
The game was Paul Lambert's 100th in charge of Villa but they are now winless in their last eight games.
A point lifted them to 16th, two points above the drop zone, and Lambert was pleased with their resilience.
He said: "I thought we were excellent in spells of the game and there were massive pluses
(Alan) Hutton was excellent and the two centre-backs were outstanding
(Jores) Okore has not played in 14 months and he was outstanding
Ciaran took the lead and helped him and Gabby was a threat the whole night."
The crowd of 25,311 was the lowest in the league at Villa Park for 15 years but Lambert backed the supporters.
"People paid hard-earned money and they are still getting behind you, that's what this team needs, he said.
"They have been great, I've got nothing but compliments for the supporters here
It's a massive football club with massive expectancy levels and like you said it was cold and on Sky."
Source : PA
Source: PA