Actually I have had many, many entries for this category over the years but I thought I'd share one of my favourites with you.
It's a Sunday and I'm working bar in the restaurant in which I am then employed (about six years ago), the restaurant is pretty busy and there are waitresses running around carrying food as normal (including smoking fajitas). The bar on the other hand is very quiet and I'm doing bugger all, as is my preference. The front door opens and in comes a middle aged business man. He ignores the hostess and heads straight to the bar. Then to my utter disbelief he says loudly to me "There's a terrible smell of food in here! Do something about it!" When my shock passes I tell him that "Um we're a restaurant, we serve food." He wasn't at all mollified and insisted that I "do something about it." I looked around, at the tables waiting for their food, the servers running up and down and then back to the guy standing at my bar. My normal bartender slick replies deserted me, what could I do? Make all the restaurant customers leave? All I had was "No." The guy looked surprised and replied "well in that case I'll leave." What could I do? I just said "Oh well. Bye!” with a cheery grin.
