I'm so glad I don't work the register anymore. All your stories pretty well match my experiences working at the pet stores.
My daughter is now working at a pet store, and she told me that one day she got to work and someone had returned a bag of cat litter claiming when she opened it, it was already full of used litter!
Yeah, right! I would have thrown them right out of the store, but the manager actually exchanged it! *rolls eyes*
I just had to exchange 2 retractable leads I bought last fall. I hated to do it since I couldn't find my receipt, but they were $35 each and I need them for training my service dog. Fortunately, I buy my food from the same place every time, and the manager had been having other leashes returned/exchanged for the same defect so I didn't have any trouble.