Last week, I made an appointment to take my car into a local shop for an oil change/tire rotation/state inspection. The owner gave me a call when it was done (except, apparently for the inspection, because I'd made an appointment on the inspector's day off. Which--huh.). He asked about the car "shuddering." I thought he was talking about how it was a little choppy when changing gears, but when I got the car back (after paying over $80 for the oil change/rotation), I understood what he meant. It was obviously misfiring, and in the three block drive back to my house, it stalled twice, and the check engine light came on.
I brought it back to the shop, he ran diagnostics (for $90), and said that the problem was with the air intake sensor on the engine, and repairs would be around $500. And this couldn't possibly be caused by the oil change, even though my car (with 6,000 whole miles on it) has never shown symptoms of this before.
The car is still under warranty, so instead, I had it towed to the nearest dealer. The problem? The oil cap wasn't tight enough. Which caused oil to leak. Which caused airflow problems. Which caused the misfiring.