I think the point is it happens less in modern animation. Having to search for errors is not the same as glaring mistakes that are impossible to miss.
I don’t really watch anime. Only error I’ve happen to catch in recent times was an episode of the Simpsons. A part of the wall clips through Marge’s hair for second.
Now that I think of it (it’s not a mistake exactly) for awhile in later seasons you could clearly see a shadow behind the characters. Their cell on top of the background. Maybe it’s more noticeable in HD but this not an issue anymore so that’s nice.