I had this happen on a ride I was on once. I announced myself before passing a guy and he immediately yanked his handlebars to the right, sideswiped his wife, and went down, fairly hard. i felt bad, but I'm not sure how I could have helped it. I had slowed and was only going about 1mph faster than them as I prepared to overtake them. Announcing yourself is still preferable to just passings someone unannounced in my opinion. It's amazing how often I seem to startle people before passing though. I never scream or anything. I think they just aren't paying attention.