On the half-hour mark, Celtic midfielder Cha Du-ri went to guide a harmless long kick from Rennes keeper Benoit Costil back to Forster but somehow the ball evaded the Englishman, on loan from Newcastle United, and found its way into the back of the net.
Forster has decided that it was his fault the goal was scored but has indicated that it is nothing to sulk about as the Hoops are back in action in the league against Aberdeen on Sunday.
"It was a bad mistake and I know I've got to do better. You just can't let things like that happen. I wasn't loud enough with my shout and it was completely my fault," he said.
"It was disappointing and I've just got to be a bit more vocal.
"There isn't a person on the planet who hasn't made a mistake, especially footballers, but you've just got to move on and try to put it behind you.
"The goal can be a lonely place, but I concentrated on forgetting it ever happened because you can't change the past.
"That's what being a goalkeeper's about. Mistakes happen - you've just got to stay positive and move on."