I’ve broken many a thing on my bikes over the years: wheels, pedals, cranks, chains, handlebars, and even the frame (on two different occasions!) But I haven’t broken a seatpost until now.
Okay, I’ve broken part of a seatpost before, namely, the binder bolt. This has happened on two separate occasions, the last time right before I was going to do a week-long tour in remote terrain. (Note to self: Carry a spare binder bolt for touring.) While sucky, the bolt/clamp break isn’t super-catastrophic, as you are still upright, just now with a very loose saddle.
But snap a seatpost? You are going down.
This is what happened on Saturday December 28, 2019. I was riding my Raleigh Crested Butte around in SE, running errands. At SE Salmon and 28th the post snapped at the seat cluster. I wasn’t riding that fast (maybe 13 mph) so I managed to slow down a little bit when I knew what had happened. But not slow enough, and I realized what was going to happen next: I was going to make personal contact with the pavement.
After the fall, I got up. Thankfully, no cars around, nor anyone to see what had happened. I was definitely scraped up on the knees and sore, but no rips. No broken bones. I wheeled the broken bike to the curb, leaned it against a sign post, and took a moment to breathe (and dig into the first aid kit for Ibuprofen.) I called up Emee and she came over in the van to pick me up.
We immediately drove over to Portland Bicycle Emporium in hopes that Geoff could solve the dilemma. I was worried that since the post broke off just slightly inside the frame, that it would be hard to get it out. Not so, Geoff got it out fast, and popped a new post in. Problem fixed!
Now there’s still the whole biz of the seatpost actually snapping, since this is highly uncommon. This wasn’t the original post, something I got replaced four years ago. Internet speculation is that the post is still undersized at 26.0 (the original one was 25.8), and over the four years the .2 mm of space meant that the post was rocking back and forth, enough to do what it finally did. We tried putting in a 26.2 post at the Bicycle Emporium, but that was too big. Of course, that may be because of how “tight” the enclosure is, so maybe a shim or something will totally do the trick? I don’t know for sure.
But breaking a seat post is another thing to worry about. Thankfully I wasn’t riding at top speed, downhill, and/or in traffic, because I don’t want to think about what could have happened…
Urban Adventure League 
Wow don’t think I’ve heard of that happening before. Glad you were ok.
I have never had this happen to me and very surprised to hear it could happen. I’m glad you are OK.
I took a second look at your installation. I notice you have your seat as far back as it will go. This provides a longer lever moment toward the rear of the bicycle. The post broke off pretty much flush with the top of the seat post tube so flexing was happening right at that point. Your post was out quite a ways but nowadays they put them out much further than that on all kinds of bicycles. However I believe a lot of the modern bicycle frames and posts tubes are of a greater diameter than your older MTB has. Glad you didn’t get hurt bad. I’m heading out to inspect my seatposts now.