I just let stuff like this slide... It's not possible for every writer to go back and watch every episode before they start the script.
They don't have to. There should be a big notebook full of all the established information (Bios on the characters, etc.) that can be checked. You don't write a story about the sister of someone after it's been established he's an only child. Look at the bible, read the chara's bio.
There's no excuse for not keeping track of that stuff.