I found a bug where if you are digging around your feet or close, you can fall through the terrain and die.
Yes, it's a known bug, it may happen in singleplayer as well as in multiplayer. It may happen more frequently on multiplayer servers, when multiple players are digging at the same time.
Only workaround: Press F2 when falling through ground, this enables you to return to the surface safely.
We're working on a solution
Ah cool. Thanks. And I just found your official bug reporting thingy. Sorry about that. I will post bugs more appropriately from now on.
Oh yes, bug reports are always nice! Especially since you're hosting your own server, if any bugs or issues related to multiplayer occur, it would be very helpful if you provide us logs or (if available, errorlog files). You can alternatively also send them as private message