On the second day of TOSRV South (Tallahassee to Albany GA and back) my Modolo stem broke right off. I came down hard on the post and fortunately didn't crack my sternum. I have heard that it can be right painful. Support provided me with a bike for the rest of the ride.
In Lancaster CA an incompetent bike mechanic didn't pin a new chain correctly and got the chain length wrong. It came apart as I was leaving the KOA the next day. Fortunately the buses there have bike racks so I was able to return to the shop and get a new one from the guy who owned the shop.
Peugeot once made a frame of aluminum with the joints pressed together - no welding or gluing. It was an interesting idea - get the tolerances tight enough and the aluminum will weld itself - but didn't work out. Mine failed at the front of one chainstay. I took a couple of spills before I figured out what was going on. I gingery pedaled back to my van and discarded the frame when I got home.