My wife refilled her nail polish remover bottle from a quart of acetone I'd just bought at home depot. A bit later she says "Has acetone been reformulated? This E36 M3 doesn't work worth a damn anymore".
I explain that acetone is a chemical and that it is what it is.
I fetch the can, pour some out and investigate.
It's so far-fetched an idea it takes a while to dawn on me - someone had bought it, used probably half of it, topped it up with water and returned it.
Another time I opened a gallon tub of cove base adhesive to find it partially used and with a plastic spreader left in the container.
I am SO glad I do not have to deal with the general public in my work..
that said, I remember being 15 and riding my bike miles to a computer store to pick up a submarine game for my C64. I get home and there is no disk inside the box.. so back I go and get a refund (needed help from dad there)
A couple of months later, they get bought out by a store in the mall. We are shopping and I spy the game on the shelf. yup.. SAME game.. no disk
I worked retail for 12 years. The only day worse than black Friday was the day after Christmas. People were always just buying something the stick under the tree. Right, wrong or indifferent, they would just buy anything, planning to return it the day after. Most of them miss the sign, in bright red letters, "No Refunds, All Sales Final". There would be at least one pissed off customer every year. We even had a guy take a brand new, never run, RC car out into the parking lot and stomp it to pieces because we wouldn't give him his money back. 
The bosses philosophy? Don't like it, shop at Walmart.
I'm glad I don't work retail anymore.
