When Lucy gets wrongly nabbed for shoplifting Maryjane is sitting right near her as the police arrest Lucy. Later, Maryjane walks in with Mr. Mooney and the same police who arrested Lucy see Maryjane once more but don't recall seeing her previously (and Maryjane is dressed the same in both scenes).

That was "Lucy Meets the Law," which first aired on February 13, 1967 (Season 5).

Pity the writers were sloppy about that rather obvious fact as outside of that it was a very good ep.