I was mining in a cave, on z-position 11 and there was a bunch of lava. So I poured water on the lava which created obsidian, and then stood on the obsidian. I mined something above me and gravel fell on top of me (like 2 blocks, enough for me to suffocate). I immediately went through the obsidian, fell through several blocks, through the bedrock and saw myself falling in the void and died. Death reason: "fell out of the world".

I went back later and found that there was no lava under the obsidian, just 3 andesite layers, followed by like 5 stone layers. It all happened really fast, no resistance, so it's hard to say what really happened but thas what I can tell from going back after I respawned.