Random Language Rant

Forgive the tiny rant, out of nowhere. I don't know why it annoys me to the extent it does.

Why do people write "give someone their due" as "give someone their do"? That doesn't make sense. Give someone their due means to give someone what is due to them, it's a word, it means something in context. 'Give someone their do' ... what is a 'do' anyway, and how do you give it to someone? (Unless you're in Ireland, where a 'do' is a party/event, and 'giving someone their do' could conceivably mean throwing them a party.)

Just ... *growls* For some reason, that mistake really irritates me.