9 Directors Who Admitted Their Movies Have Plot-Holes
4. James Mangold - Logan
The Plot Hole: It's firmly established in both the comics and the X-Men movies that Charles Xavier (Patrick Stewart) loses his hair - due to his psychic powers and an encounter with Apocalypse (Oscar Isaac) respectively - so why does Charles have a faint head of hair in Logan?
Writer-director James Mangold has acknowledged that many fans have raised the issue to him, and suggested a possible explanation that Charles has grown hair because his psychic abilities are beginning to slow down with his advancing mental illness.
It seems like a pretty flimsy excuse for something Mangold likely overlooked during production, honestly, and doesn't fit at all with any prior established explanation for Xavier's baldness.
You can argue that Logan, being a mostly standalone, self-contained movie, isn't bound by any pre-existing continuity, and Mangold's workaround is an efficient enough solution, but it's also unmistakably cobbled together after the fact.