With just over 20 minutes remaining, Reading led 2-1 when centre-half Cahill tested his luck from just inside 30 yards and it appeared as though Federici had the ball covered until it took a slight dip and ended in the back of the net.
But just as he did last week, Royals boss Brian McDermott was right behind his gloveman despite another error in the 4-2 away loss.
"For the second goal it has moved, I'm not concerned about it at all," McDermott said.
"He has a fantastic mentality, and for all of us, when a result has happened you box it off because you can't affect it. It was one of those things, he had a shot, it moved and it went in.
"That's life, nothing you can do about that now."
Chelsea's third goal, scored by Fernando Torres, was controversial as the Spaniard was offside before the Blues were gifted an injury-time fourth to Branislav Ivanovic after Federici had bombed forward in search of an equaliser.