My 12 year old has been using this excuse lately. I forgot. Tonight her and her cousin run through the house and into her room, minutes later they run back out the front door. I thought nothing of this, they do it ALL the time. I was putting some clothes away and went into her room and her window was wide open and her screen was out and sitting by her bed, letting every flying bug in the county in my house! Hmmm....now, they didn't sneak out, they walked right past me. What were they doing? I called my brother's house and made her come home and what did she say when I asked her about the window? "Oh, we were doing something and I forgot to put it back."
I then asked her if she had fed the dog yet...and she said, "I forgot", I swear to you, I wish I was kidding. Btw, now that I think about it "I forgot" was one of her Dad's favorite excuses too.