Star Trek: 12 Biggest Ever Retcons

In The Changeling, the Nomad probe comes aboard the Enterprise to investigate the crew. Its methods of investigation are dangerously destructive, though, as seen when it completely wiped Uhura's memory just to learn what music and singing were. There was no brain damage, but she had lost all of her knowledge and memories, requiring her to be re-educated.

Such a drastic action is never brought up again. In later episodes, you would never even guess that Uhura had lost all of her memories. Re-education is one thing, but Uhura had lost all memories of her childhood, of her hobbies and interests, and yet everything seemed perfectly normal.