Sometimes Moviebase lists an “Up next to watch” episode that does not exist. Here are two examples:
Wednesday displays an “Up next to watch” episode 9 in season 1, but as can be seen on the Season 1 entry below, there are only 8 episodes.
Will Trent displays an “Up next to watch” episode 14 in season 1, but as can be seen in the Season 1 entry below, there are only 13 episodes.
This happens when someone adds a non-existent episode to a season in TMDb and then a TMDb moderator deletes it. Moviebase does not pick up the fact that the episode has been removed for purposes of the “Up next to watch” display. It can clearly see that the season no longer contains that episode, but the “Up next to watch” display is not updated until more episodes are released in the following season.