Watch at the beginning of the movie when Coop loses the Denslow Cup. When Ms. Reed wants him to sign her balls you'll see she walks over to him with nothing in her hands, and then when he thinks that he wants her to sign her, well you know, she suddenly has two BASEketballs.
Wasn't that the scene where we were to assume she meant to sign her breasts? So, if we had seen the balls beforehand, would we have even thought of the breasts, not the balls? I'm not sure, but, I think this was supposed to be a comedy...
Remember the laying carpet joke in there, surprisingly enough, they didn't have sex, well I'll be!
It was NOT a slip up because if they were making a movie they would have to stop the camera and run the balls to her so they'd think "oh she would have to walk up with it" and they would re-film it again!