QUOTE(melech @ Dec 24 2007, 05:40 PM)

What if you daven ma'ariv early, like from plag?
That's why you don't... (at least not any ashkenazim I have seen)
QUOTE
Do you have a source from shas, rishonim or even shulchan aruch? Because it seems to me throughout shulchan aruch, at least as far as the Rama is concerned, if you daven maariv, it's the next day for all intents and purposes [sort of; there are exceptions like doing melacha on motzei shabbat]. Ignoring special cases like sukkah issues.
I'll be honest, I didn't look...