I once had an aluminum stem break on me. I'd just finished a somewhat technical descent up to 40 mph, was on the flat and crossing RR tracks when I felt the stem get wobbly. I sat up, took all the pressure off, and managed to ride the thing another 10 miles. Weld failure. I admit to enjoying sprinting and really pried on the bars back then. I also had a steel bike, threaded stem, which felt like I was going to rip the bars right off the bike. Good thing I never did. I also had a stem bolt break in the parking lot before a populaire, different stem. I was doing my usual "torture test everything before the ride." Bontrager later recalled that stem. I drove home, got my spare bike, came back and started ~59' late, still managed to finish in the middle of the strung-out riders. Those were good days. I don't break anything anymore.
I wrote to the broken stem's manufacturer, explained how it had failed, no answer. Not on the market anymore, anyway.
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