I've got a total of 6 hours on the game, and this has already happened 3 times!
I go to dig a ramp, so that I (or more recently, my truck) can get out of a hole... and what do you think happens? While attempting to form the floors and walls around me, I somehow slip through them and wind up in a cave, far enough down that I instantly die on impact!
I've never been able to recover the items, and I suspect it's primarily caused by the horrifically low frame-rate my game is forced to play with (seems like about 10-20 a second, most of the time, and nearly 5 when there's a storm)
PS