Wycombe thought they had scored early on when the ball bounced off one of their players from 30 yards, and went over Griemink into the back of the net, but was ruled out for an offside.
Wycombe scored 1 minute into the start of extra time to leave many Swindon fans thinking why couldn't they have done that a couple of minutes earlier so we could have all gone home.
Some fans at this stage in the Town End began to sing "We want King Out" as this leaves Swindon having lost 5 games in a row, and if they lose at QPR if will be their worst run since 1921.
Adam Willis put Swindon back in with a chance with his goal on 100 minutes with a close range goal beating former Swindon keeper Frank Talia after a Gurney free kick.
Wycombe booked their place in the 2nd round with 10 minutes to go with a close range shot from McCarthy.