12.) “3rd Rock from the Sun,” “The Thing That Wouldn’t Die” (2001)
In the final episode of this sitcom, Dick Solomon (John Lithgow) ends up having to go back home to his original planet after turning another alien into a monkey.
Dick then reveals to his love interest, Mary (Jane Curtin), that he is an alien. Initially, she’s interested in joining him on another planet, but she quickly realizes she is not capable of such a drastic change to her life. Dick then decides to erase all of their memories together by hitting her in the neck and knocking her out cold before he returns home.
There are plenty of ways he could have erased her memory; seeing him knock her out after they’d had such a good relationship was unexpected.