Other mistake: Lynette says "I also have a 9 year old", but on Penny's birthday not long before that she turned 11, and the only younger sibling Penny has is the baby.
Suggested correction: And in the episode with Penny's 11th birthday, her parents forgot it was her birthday and in the last minute of planning, her birthday cake said "Polly." Hard to say if it was intentional to show what kind of mother she is or accidental/bad script writing.
Literally addressed in the show, she said she put the wrong name because they had been discussing the name for the baby.