I was getting ready to play Minecraft just like usual. I had the game opened up, and my survival world was loading. Everything was going smoothly until my character's spawn... or lack thereof. It's kind of hard to explain, since I was there, but the game didn't seem to recognize most of my actions. The world was mainly transparent, with only the sky and a few cave systems (and my player's discolored arm) visible on my screen. I started falling through the world slowly, as if I were riding down on a waterfall.
As I fell, cave systems would appear, then vanish in an instant. When I reached the void, I stopped falling, and I found I could walk. commands didn't work even with cheats, anything I typed in the chat didn't appear on screen after I pressed enter, and I couldn't jump. As I moved, the caves started glitching out again just as they did during my descent, and the day-night cycle moved normally throughout this entire ordeal.
I honestly don't know what triggered this. I joined the same world not three hours before, and I didn't have any problems then. I can look at my coordinates, and all of the "F1" functions seem to work just fine. I paused the game, and after I returned, the whole screen (save for my slightly discolored arm) was black. I took a few screenshots using F2, so I have those available if you want a visual.
I don't know what happened. One moment I was having a sip of coffee and thinking of all the hijinks me and my dog Sir Boofles the fourth were going to have down in the mine, the next, I was sinking down into the world.

What I expected to happen was the usual spawning experience, without the world caving in beneath me.

What actually happened was I fell through the world without provocation, without even moving an inch.

These are the only steps I can provide to trigger the glitch:
1. Join your survival world.
2. Allow yourself to fall through the floor.

This is on the PC, by the way.