I once had the front lug nuts loosen up on a VW I'd bought about a week earlier. Fortunately the odd clunking noises alerted me to the problem before a wheel came off.
Just a few days ago I had the left engine mount center bolt almost fall out on a used Honda. I'd noticed it looked a little odd when I looked the car over before buying it, but then it slipped my mind. I think in the future I should write stuff like that down instead of trusting my memory.
Generally every time I go looking at used cars I run across at least one that seems like a genuine threat to life and limb. Experience has taught me to stomp hard on the brakes and also make sure the driver's seat is actually anchored to the floor before setting off on a test drive.