I read of one person's experience where this guy was out of the country after the interview, and oath letter arrived. This was a week after he left and so he missed the oath ceremony. He got another letter from USCIS after another couple of months stating that since he missed the oath ceremony and didn't inform USCIS, he abondoned his N-400 application. By the time this guy came back and worte letters to USCIS, it was too late and USCIS still refused.
Just remember...you are still a LPR until you take the oath!