After much arm waving and shouting at double glazing, this SC realised that people in the store can actually hear you if you go into the store too. After two false starts she even found the door.
SC: "That pump's not working, it just won't pump."
Me: "Maybe you're doing something wrong, we haven't had any problems today."
SC: "no I think the pump's broken. I'll move to another one..."
(out she goes, moves to another pump, and what do you know, that one's "broken" too. On re-entering
SC: "You really have to do something about those pumps, they don't work."
Me: "This is the first problem we've had today..." (about to say "if you can wait a minute I'll get somebody to come out and help you")
SC: "Well I guess I'm just a stupid old woman then!" (stomps out)
The best part is that since she said it, she can't complain about me saying it.