I often use the wrong, but related word. So I just now said 'pudding' instead of 'breakfast', and I might say 'night' instead of 'day' or 'tomorrow' instead of 'today'.
I don't notice this until someone points it out. It's got worse recently - I think it's been worse since having DS (16 months), who still doesn't sleep through, and I think it's worse in the evenings as it's usually DH who points it out. I therefore think it's due to tiredness. However, perhaps I do it all day long and no one else mentions it! Does anyone else have the same problem?