At Central Perk after Phoebe has unwrapped the salt water taffy, she holds it in her left hand, but in the next shot it's in her right hand.
When Ross first opens the box of salt water taffy he sets the lid down on his leg, when the camera angle changes he sets it down again, this time on the table.
Rachel says that Rattus norvegicus was responsible for the plague. The species actually associated with the plague was Rattus rattus (though it wasn't, in fact, responsible for it either).