Worst..... glitch..... ever.

so I spent the WHOLE day building my fort, consisting largely of a long wall with uncoordinated windows. it was looking amazing. I run out of cobblestone to make my smooth stone, no problem i'll just go mining for abit. Half an hour or so later the game suddenly closes I think "i hope it saved not too long ago, I dont want to have to collect all this stone again"...

I start it up, im in a similar position, maybe a few fewer stones dug out. feeling fine i return to the surface, open the door, stop, look around. something's off. OH! i see, the whole of the fort i've been building all day has disappeared for some reason... great.

what's the worst glitch you've encountered?