The Last Word

Yes you are.

I know you are, but what am I?

Not with the last word.

I think you put the word "not" in there by accident.

there's no not here, only last word.

Say what?

There's nothing to say but me having the last word.

That would make you a liar.

I'm afraid you're both mistaken.

Only one is mistaken and it's not me.