Not the thinnest but I had a 99 Transit 2.5 in years ago for the first time since my customer had bought it. He just asked for oil and filters to be changed and to have a look in at the brakes.
After advising that the belt should be done if he had no history of its replacement, I pulled the covers. I couldn't believe it was still running. There were teeth missing everywhere along the length of the belt that had collected in the bottom cover yet it hadn't even jumped. No camera phones back then so no photographic evidence for you, but I swear it's true. Not the worst engine if the belt went admittedly, but inconvenient all the same.

Liam