In the episode that Bruce Willis takes Rachel to the cabin and they bump into Ross & Young Daughter of Bruce Willis, watch Rachel holding her glass. First it's in her left hand, then her right, then her left, then her right again. Minor, true, but nonetheless a slipup.
This is true, Ben; people don't always hold objects in one hand forever. However, watch the scene I'm talking about and it's pretty clear that this is a slipup.