First off, I doubt I'll be in line behind a robber. The first thing a robber will do is get in the back of the line, or come when it's less crowded. The second thing he will do is clear the immediate area around him.
Oh wow! I guess you've never seen the recorded video on TV where this exact scenario happened, except within a convenience store. It was a Discovery Channel special, or something similar. BG rolls up to register, points gun at teller, while the plain clothes officer is standing right behind him. He freaked (the cop). Immediate draw, stuck it to BG's head, and gave him the ultimatum. BG couldn't believe it! Dropped his weapon on the counter, and so it goes... In this incident the narration points out that often BG's get tunnel vision when doing their crime and are very unaware of their surroundings.
It's funny, because after watching that one video, I believe we have a much better chance of being confronted in a similar way, rather than a home invasion for instance.