I think you have a similar problem to the one smallwhite pointed out. I thought about this one a long time myself, because I don't think it is wrong, it just sounds a little odd to me.
I called a waiter who was over there.
You're talking about a non-specific person, then you give a specific location. While the sentence isn't wrong, it would sound more natural to have "the" instead of "a". However, if the sentence was longer and with some punctuation it would be more natural.
I called a waiter, who was over there, to bring me the bill.
But take this with a grain of salt, although I'm a native speaker I frequently make mistakes in English.