This slip up happens in the episode Father Of The year. Marcia is writing a letter about Mike to the newspaper for the father of the year contest. While she is in the middle of writing it, Mike,her stepfather, punishes her for not doing her chores and she gets mad at him and you can clearly see her crumple up the paper in which she has started to write the letter and throw it in the wastebasket. That is in one scene. Later, she has a change of heart and decides to finish the letter and mail it. She takes the crumpled paper from the wastebasket, finishes writing it and then immideatly seals it and puts it in the mailbox to mail. Watch the next scene, where they show Hamilton Samuels, publisher of the newspaper, reading Marcia's letter. He has the letter in hand and then puts it down. You can see that the paper is not crumpled,as it should be.