Responses from the /users/[username]/watched/movies endpoint seems to result in the incorrect profiles films being returned in certain cases. It's happened for me for users with usernames that contain only numbers. This could potentially be due to the API fetching the watched films of the user with the User ID corresponding to the number that's actually the username.
e.g. "2" is a deleted Trakt account. https://api.trakt.tv/users/2/watched/movies responds with lots of movies from 2026, which isn't possible. It might be trying to fetch the watched films of the user with the User ID 2.
This doesn't seem to be an issue with the /watched/shows endpoint.