After getting tired of my "starter cave" base I figured it was time to move out to an actual base somewhere else, so I make a few backpacks, grab everything I have and as I am about to move out I see a bunch of red dots on the map, in an enclosed space near where I'm at, I figure "Hey may be a dungeon or something, might as well check it out while still here."
I tunnel in a stair patter there, get low enough so that the floor starts looking dark...too dark, I move in to place a torch only to realize there was no floor...
I fall in to a cave with angry zombies and creepers.
Zombies kill me, Creepers explode everything I dropped.
Since I had packed everything, means my bed as well, meaning I respawn back a original spawn with no idea how to get back there.
Moral of the story: DO NOT explore underground when you're carrying EVERYTHING you currently have in that world. T_T
Also, a lot less of a "Oh no..." moment and more of a "Good gracious, what have I done?!" moment:
Basically, Wand of Equivalent exchange + infinite Nukes = Crash and all land destroyed down to bedrock as far as the eye can see once I could get the world working again.