True story...
Western Auto....customer came in and got oil filter, and oil. He called a little while later, and said the oil filter was different then what was on his car. Different shape and size, would not thread. I told him to come on up and we would switch it out for the right one. An hour later, he showed up, with the new and old filter IN HAND. He said he had to walk the rest of the way when his car quit running.
He actually tried to drive back up with the filter off, and locked the motor on the drive back to the store.
I felt so bad that he had locked his motor, but there was a certain assumption that if someone is coming to the parts store to get stuff to change their oil, they understand the basics...
