There is no instance in which the original words could possibly be used to someone who was there at 8am.
I'm going to play devil's advocate and propose a very narrow and unlikely context where it might. However, I do agree with ems. I just can't resist a "no instance" challenge with language.
You turned up at 8:00. You're angry about it because once you showed up, there wasn't anything to do for whatever reason and you wasted your time. You angrily ask your boss "Why the heck did I show up at 8:00 then?"
Your boss shouts back "You were
supposed to be here at 8am!" (i.e. "because I told you so")
Not very likely or natural, and also requires a shift in emphasis, but it does allow for the original words to still be said if someone did show at 8:00.