the threat from being the last one in a group of people going down a dimly lit alley (to be ghost bummed). While you do not actually see anything behind you, ghost bummer may appear from nowhere and 'take' the last person in the aforementioned group. Therefore, it is never good to be last. Also a good motivational technique to make lazy people walk faster to a bar.
"quick! walk faster, or ghost bummer will get you! fatty"