The worst was my great aunt's Oldsmobile Calais. It was a late 80's w-body version and I took it into the shop to install some air shocks for the rear. She was having a electric scooter carrying platform installed and needed the shocks installed on the car.
After I finished the car, I thought it would be nice to wash and detail the car for her. Her niece and nephew had been using the car mainly and it was filthy.
I found chicken bones and other fragments of food on the floor. The paint was matted with pollen, dirt, and all kinds of crud. The interior smelled horrible and the carpets had all kinds of crap ground into it.
24 hours of total work went into detailing the car. I even buffed the head lights back to perfection.
She cried tears if joy and hugged me when I brought the car back to her the next day. She said she hadn't seen it that pristine since it rolled off the show room floor.
Crazy thing was I did that back in 2005. The car was an 1988 year model car I believe. It only had 35,000 miles on it as she rarely drove it. After getting the crud off the car the paint buffed out to a perfect finish.