I had two close calls: One time my down tube developed a large crack, starting from a faulty weld to to the head tube. Luckily, the lost stiffness made the bike

I had two close calls: One time my down tube developed a large crack, starting from a faulty weld to to the head tube. Luckily, the lost stiffness made the bike start wobbling so much that I got off to inspect it. On the replacement bike, they didn't tighten the Hollowtech crank arm enough, so it slowly slid off over a few months and finally fell off. I'm glad that I was away from traffic and in my saddle at the moment it happened, so I could just walk the rest of the way home and put the arm back onto the crankshaft.