Most stupid and life-threatening..
I was working on a concrete balcony, with a wooden railing on steel posts, which at the bottom are welded to horizontal steel pegs (hollow, as it turns out) embedded in the concrete. I had disconnected the railing sections from each other to repair and paint them, and the section I was working on was about 1m long. I was leaning all my weight on it, trying to reach a bit of the post near the bottom on the outside, when the whole thing started to give way. The pegs, probably weakened by rust under the paint, were bending surprisingly willingly. Somehow I managed to pull myself back before crashing head-first onto the concrete below. The sturdiness of the posts was very misleading.