John Roberts Motors (Chevrolet Dealer) in Gunnison, Colorado was asked to change my oil and replace spark plugs. They checked my air filter and pronounced it "good". Three days later I discovered at another repair facility that they had left the (dirty) air filter cover loose and hanging in the breeze.
When John Roberts secretary called asking how was my service, I told her. The new owner called and told me that if I asked specifically, he would refund the portion of my bill related to checking the air filter. I declined to "request". He thought better of his position later, and reifunded the entire amount of the bill. I later found (at another dealer) that John Roberts' people had also failed to properly tighten the replaced spark plugs.