From  u/CappuccinoBreve  Tales From Retail:

I lost it with a "serial returner" once. I'm typically very calm and friendly, which is why I'm often called to handle difficult customers.

This customer was making a scene with one of our new cashiers about returning a bunch of plants, no receipt, most in pots with labels showing they weren't purchased at our store.

Serial Returner: "Just do the return! It's fine! It is not a problem! Just do the return!"

When I got to the register the cashier had started to cry so I sent her to the break room to calm down and told her I would handle it.

I asked if she had a receipt.

Serial Returner: "No but it is fine! Just do the return!"

Me: "Ma'am I'm sorry these plants weren't purchased here."

SR: "No it is OK! Do the return! It's OK! "

Me: "No it's not, the labels says you bought them at BigBoxStore"

SR: "OK fine! "

She then pulls the plants out of their pots and slams them on the counter sending dirt everywhere!

SR: "There, now it's fine! Just do the return! "

I lost it! I raised my voice...

Me: "No it is not fine!"

I stuffed her plants back in their pots, brushed all the dirt I could off the counter towards her. She jumped back.

SR: "Oh! Why are you so upset!"

Me: "Because you made our cashier cry and very disrespectfully dumped dirt all over my counter! Now I will return these two plants over here with our label on them for store credit only, you need to take these other plants back with you!"

SR: "Oh... well ok."

I gave her her store credit and sent her on her way.

My coworkers were wide-eyed because I really never lose my cool like that in front of a customer.

Serial Returner has yet to darken our doorstep again.

--u/CappuccinoBreve