"Do you only hire stupid technicians?!"

I was quite upset at this so, especially after having explained to the customer that his issue was in no way related to the services we had provided him. I told him that there was no need to be rude or offensive. He said he didn't find what he said rude or offensive, so I asked him, "...if it'd be offensive if I stated that we had only stupid customers."

Shockingly, he was upset and demanded a refund. :p