Apart from physical violence, sexual inappropriateness, or being openly bigoted, the fastest way to get me to dislike you is to say "Well if there's no price on it, it must be free." when I'm trying to ring you up.
A guy who I had been having a great conversation with said this today and saw my face drop.
Guy: WOW. Did I just annoy you or something?
Me: That joke is not, has never been, and will not ever be funny to anyone who has ever worked retail. You should never say it.
Guy: Is it really th--
Me: Yes. It's really that awful. Occasionally, a clerk or manager will smile and politely laugh with you, but if you pay attention you will sense just how much they actually hate you by the grind of their teeth. Never say it again. Ever.
For real, though. Never say it.