I've had this the other way round. I was in Basel airport at 6am or something stupid last month and wanted to grab some mints (travel breath) and a red bull, so popped in to the shop right next to the gate.
Went to the till and the woman was on her phone to a family member (long enough for me to glean this!) I sort of lifted a finger slightly as to get her attention, not in a rude way, but to signal I was ready to pay. She responded with a loud sigh and said to the other party she would have to go as someone was hassling her.
I asked to pay by card, she said no. I was like, err OK, why? As she offered no explanation and I could see the card machine, like most shops in the world. She snapped back "It's broken, are you calling me a liar?!" I was like, no I was just curious as it's on, no errors etc. can you use the one from the other till? (I really didn't want to spend my cash)
She went nuts at me and said that she was refusing to serve me as I was challenging her?! I was proper calm up until this point, when I pointed out the fact that she was on the phone when I came in, obviously not a work call, and that she had been rude from the outset.
She then picked up the phone shouting at me; "COMPLAIN THEN? CALL MY MANAGER!!!"
I couldn't be chewed at that time of the morning, went round to the cafe and bought a beer. Airport morning beers ftw.
Cool story bro etc.