When applying a third coat of paint do I have to do the whole wall or just...

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...the bits that are missed? Stupid me painted when the sun was coming through the patio doors onto the wall I was putting a second coat on, making me miss bits and pieces here and there. I was wondering if I could just touch up the middle of the wall, where it looks bad, or if I have to put a third coat over the entire wall? I'll do the whole thing if I have to, but I have a baby and whatnot so it would be easier just to touch up the rough bits. What do you think?