I picked up a Diamondback Sorrento that I found on Craigslist for my oldest son to use as a commuter bike.
It looked good, except there was some rust on the chain and cluster.
It cleaned up nicely, and I took it for a spin up and down the street in front of my house. Everything checked out on the first pass, so I sped up and shifted up. As I stood up, the chain broke, and I ended up sliding to a stop on my face. No serious injuries, but I had to get my nose stitched up in the emergency room.
In 40 years of cycling this is the first time I have seen a chain link break.