Towards the end of the movie when Monica is coming home from her new job she is wearing a pair of heels. When Quincey laughs at her because she doesn't know how to walk in them she takes them off and carrys them. After she has a conversation with Quincey she walks away in her shoes that she clearly took off as she got home from work!!
If my memory serves me correctly, I think she puts them back on during the conversation. Not trying to prove you wrong, I just think that's how it happened.
I basically have this movie memorized because its one of my favourites but I re-watched that part of the movie and it is a slip-up. When she gets out of the car and goes onto the grass, she takes off the shoes. When Q approaches her to talk you can already see that the shoes have disappeared from her right hand, and after the conversation she walks away with them ON. At no point during the conversation does she put them back on.