Something like this almost happened to a flight of mine last year in SMF. The catering folks came up to the plane after they armed the doors (and I presume the anti-collision light was on), and tried to open it.
I've never seen the FAs move so fast to disarm and hold the door shut. I couldn't figure out why catering was coming up to our plane when we were all buttoned up.
I hope that catering guy said thanks to his preferred deity that night, because he came within six inches of getting killed.