When has someone "popped up" in the right place at the right time? 

We hear stories all the time how people show up (seemingly out of nowhere), help you, then sort of 'disappear'.  

It happened to Marissa when her dad had an emergency on a plane.  A doctor showed up, helped him, then when they went to thank him...he was gone! 

Has this ever happened to you? We want to hear all about it!