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This appears to be another example of how what seems like a solid robbery plan doesn't survive the first few seconds of contact with reality.

I can't make out exactly when the shotgun was fired from the video.

It looks to me like the shot may have been fired due to the victim's struggle for the gun while the bandit's finger was on the trigger.

This would make a fair amount of sense in that shooting the clerk as a BG's first action is not the best way to get away with money.