Alex Pearce had put the Royals ahead against the run of play in Saturday's Championship encounter before Federici - who was outstanding in the first period to keep Forest at bay - made a horrible mess of a clearance to allow Wales international Earnshaw to pounce.
Brian McDermott's men have yet to find the rhythm which made them a potent force after Christmas last season and in the end were glad to share the points with their much-fancied visitors.
Forest midfielder Paul McKenna injured himself during the warm-up so Poland international Radoslaw Majewski came into the line-up.
And Majewski was first to chance his arm with a long-range effort which was well saved by Federici before the hosts were also forced into an early change.
Right-back Andy Griffin lasted less than nine minutes before limping off to be replaced by Brynjar Gunnarsson and it was Federici who was called into action again moments later to deny Earnshaw.
Forest had controlled all the play but it was Reading who stole ahead against the run of play in the 15th minute.
Marcus Williams delivered a free-kick from wide on the right and when Jimmy Kebe rose to head back across goal Pearce was unmarked to hammer in a volley from close range.
Forest countered sharply and Paul Anderson was the latest to be denied by Federici who held his header, before Jobi McAnuff scuffed a shot wide at the other end as the half came to an end.
Forest would have felt hard done by at the break but they rallied well to level in the 50th minute in comedy circumstances.
Federici has made a habit during his Royals career of starting counter attacks quickly by launching kicks downfield but the Australian made a real hash of his effort to gift Forest a goal.
Federici made a routine claim from a cross and looked to smash the ball up the pitch but only succeeded in kicking into the back of defender Matt Mills.
Earnshaw was quick to react and after dancing round the prone goalkeeper tapped into an empty net.
Anderson fired over from 20 yards as Forest pushed for a first win of the campaign while the best the home side could muster was a free-kick from Gylfi Sigurdsson which drifted narrowly over.