Forster's mistake led to Crystal Palace opening the scoring as Southampton lost 3-0 at Selhurst Park.
Ronald Koeman returns to his former club for the first time since leaving in the summer - here is how you can see how he fares.
Southampton could qualify for the knock-out stages of the Europa League this week.
England play Spain at Wembley tonight in a friendly, and here are the Premier League's Spanish XI, with players from Chelsea, Arsenal and Manchester United.
The former Saint is currently in some of his best form.
The Southampton star is very impressed with the English players at White Hart Lane.
It has been quite the month for Sam McQueen, with only the in-form Romeu preventing him being named the club's finest performer in October.
The Saints' midfield general is becoming an integral part of Claude Puel's setup.
Both these sides fell to defeats in grand European venues in mid-week and will be hoping for improvements on Sunday.
Unless you rank high on the scale of lunacy, you're never sure what goes on in dressing rooms. Though Romeu reveals some intriguing info about Puel.