5 years ago I was cruising along in the drops on a flat, straight road. Luckily solo and with no traffic around. With absolutely no warning my handlebar snapped clean off, right next to the stem. What happened next is a bit of a blur but I ended up in a ditch with most of the skin missing off one knee and an extremely badly bruised sternum - I assume that it slammed into the top of the steerer tube as my weight collapsed over the front of the bike. PT said it was amazing I hadn’t broken anything and I had some extraordinarily painful rehab sessions having my ribs put back into their proper places.
Bike had never been crashed, there was no visible sign of cracking, so I guess it was just metal fatigue and the cracks were hidden under the stem, maybe over torqued as well. They were quite old aluminium bars (about 10 years I think) and were narrow diameter for their whole length, unlike modern bars which bulge in the middle and have a large clamp diameter. I have been much more diligent since then about using a torque wrench and regularly inspecting my bikes for any signs of damage or fatigue!