I guess the worst used car story I have is when I bought a "jap crap" used Ford Laser diesel. This was in the early days when Kiwis thought they had a nice late model car...which turned out to be a really neglected car.
Just bought it as a second car to replace my Suzuki 413J
I had it for about 18 months and then decided I did not need it, this is when I discovered theproblem.
I had a friend help me clean it up for sale. Strangely he could not get the finger marks off the the speedo window.... when I looked closely I could see these marks were on the inside. I reckon, but no proof that the shonky dealer had wound the clock back.....either in Japan or in NZ.