By far, the customers I dread dealing with the most are the ones you simply can't communicate with... and, for whatever reason, you really need to communicate with them. No, I'm not just talking language barriers.
Setting: Regal Theater. For those not familiar with it, Regal has a loyalty rewards card which, among other things, gives you coupons on Mondays and Tuesdays which you can only use on that day (i.e., you buy a ticket, you also get a small popcorn for two dollars)
Customer: Elderly gent. Regular customer. Very hard of hearing (but he's never asked for an assisted listening device... huh.)
He saves up those Mon/Tue coupons and tries to use them on off-days, usually months after they've been printed. It's a pain and a half to try and explain to him that the bulk of his coupons are expired because, again, he's very hard of hearing. And when he finds out he can't use them... he puts them back in his wallet. And the cycle repeats.
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There never was a post. Swamp gas from a weather balloon was trapped in a thermal pocket and refracted the light from Venus.