With the game finishing 1-1 after 90 minutes, the Australian inexplicably let a tame Alexis Sanchez shot slip through his grasp in the sixth minute of extra-time and could only watch on as it trickled over the line, allowing the Gunners to book a second successive final appearance.
The 30 year-old, who was "inconsolable" after the match, has received unwavering support from his manager and team-mates but issued an apology to Reading's fans.
Steve Clarke's men, in the lower reaches of the English Championship, forced extra-time after Garath McCleary's second-half volley cancelled out Sanchez's opener.