Factual error: There are many factors concerning Niles' post-operative state which are very inaccurate. After open heart surgery it takes at least three months for the chest bone to heal, Niles would not be able to drive a car for weeks, nor would he be able to lift or twist which he does on the show. Also, he doesn't sport a scar. Your chest bone begins very high up, just below your neck, his scar should have been visible on numerous occasions.
Suggested correction: He has a scar in season 11 episode 8. When he's naked in the cafe. A scar just like my dads from heart surgery.
While this mistake points out many different factors, I don't think this should qualify as a valid correction. The mistake is saying in season 10 he doesn't have a visible scar when he should and the correction points to seeing a scar a third of the way into season 11.