The 26-year-old France international struck his team's second goal in their barely believeable 7-5 victory over Brian McDermott's side at the Madejski Stadium on Tuesday and he has since owned up to his comical mistake at the end of normal time.
"I thought in the cup competitions in England when you draw you then replay. That's why I threw my shirt into the crowd" Giroud told reporters.
"I had no idea and I only realised when one of our subs ran over to tell me. Luckily the man gave me back my shirt so I could carry on. I have to thank him."
Despite a potentially disastorous night turning into an evening to remember for the club's 4,000 travelling fans, Giroud has urged his team-mates to cut out the defensive errors if they want to take anything away from their upcoming game against Manchester United at Old Trafford this weekend.
He added: "It was an unbelievable game and this comeback was good for our confidence.
"But we know we must correct this. We came back into the game but it won't be the same next time.
"It's difficult to come back from 4-0 down so you have to concentrate from the start of the game."
Since making his move from French champions Montpellier in the summer, Giroud has struggled in front of goal and has netted just three times in 14 appearances so far this season.