I have professional contacts who are practicing/observant Jews. I am not Jewish. Is it improper/bad taste if I say to them "Shabbat Shalom" on Fridays?
It might depend on the context, but in general, I wouldn't recommend it.
On a related note, there are times where non-Jews that recognize me as Jewish attempt to greet me with "שלום!". Although they're just trying to be friendly, they don't understand that "good morning" etc., would be more appropriate.