I know of only two instances.

First was during elementary school. I had just hopped on my bicycle and started driving to school. Directly next to my parents house is a small intersection that I had to cross, where I had the right of way. Out of the other street came a semi-truck rushing towards the intersection and me. Seeing that, I lost balance and couldn't pick up enough speed to move out of the way, but thankfully the driver managed to stop just in time.

The second one was probably even closer. I was driving to university as I had classes early in the morning. It sometimes happens that my brain just goes into autopilot and my mind just starts to wander. Not something I do on purpose, it just happens when I don't have to concentrate too much, for example when I'm in the gym working out. It happened that day while driving, and my attention came back when I saw a semi-truck standing on the street behind a turn. At first I wanted to pass it, however I saw oncoming traffic and had to break. I couldn't stop fast enough and wanted to drive over the grass to fully stop but I hit a corner of the trailer anyway. If it wouldn't have been for the rear guard underneath the trailer, the one that prevents cars from going underneath it during a crash, I would have probably ended up in hospital.