A Boy Falling Out of the Sky (a.k.a. Shifts Happen) - S9-E15
Plot hole: Dr. Lewis is talking to a patient about pets, and how she is "banned from a pet store" because she isn't good with pets. She explains that the only pet she ever had was sea monkeys as a child. But, in the earlier seasons, she had a cat. We actually saw the cat at one point when her sister was staying with her and let the cat out of the apartment by accident.