I worked retail at Wal-Mart for almost 4 years in the electronics department. We saw lots of shameful things come back on our return cart. People would buy these gigantic 52" Projection TVs for Superbowl Sunday, we'd make a lot of sales, but we'd lose them all come Monday because the same people would return the TV saying it wasn't what they were looking for.
People would buy computers, gut the innards, and return the product with bags of cat litter inside the towers.
I've seen new laptops swapped with old infected out-of-date laptops.
And we always got TV's that looked and smelled like they were in a trailer-trash smoker house, like baked ham and old cigarettes.
I almost got fired for exploding on a group of customers because they returned a ridiculously heavy and large 50" Plasma HDTV because they didn't measure right before they bought it. Now, that's a legitimate return, but it's common sense that you make sure the TV fits before you go through the hassle of having a salesperson grabbing it from the top rack, rolling it up front, loading it in your vehicle, just to have us rip the box apart, look at everything, lift the TV several times for inspection, and finally roll it back to the department and throw it up for a clearance TV.
I wouldn't put it past people to do the same thing with oil and filters, but it still amazes me how people can be.
I am extremely frugal and if say, a bottle of dandruff shampoo doesn't work for me, or some mouthwash is really disgusting, I will return it, in a very polite manner, with only the little bit I tried missing, and ask for a different kind. I wouldn't use the whole bottle and say it didn't work, as I've seen happen there.
Sometimes people are just crazy rotten. But we live in a fallen world!