Oh, I have several.
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I was exploring this really deep cave once. I mean, REALLY deep. Like, "50 foot drop ahead so watch your step" kind of deep. I was completely decked out with diamond tools, ready to take on anything that stood in my way.
Things were going well for a while, then I heard them while I was standing on the ledge of an other big drop. Bones clanking, snarling. And lots of it. There must've been 4 or 5 zombies and skeletons each down there. I jumped down to a lower ledge to get a better look. Sure enough. 3 skeletons, 4 zombies, AND several creepers.
I began to question the wisdom of jumping down there and fighting them, and was about to climb back up and get the hell out of the cave when...
An arrow came flying and nailed me in the face. I lost my balance and fell into the pit where all the creepers promptly exploded at once, instantly ending my life.
Utterly pissed off, I returned to the cave upon respawning, to find the same number of monsters in that same area. Of course, all my tools and such were still down there. I had no way of retrieving them without pointlessly suffering a similar horrendous death.
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And more recently, I had downloaded a map viewing program just for the hell of it. Delighted that it showed me the location of several nearby gold deposits, I used a GPS program to get my in-game coordinates and found exactly where I had been told they would be.
With my trusty steel shovel and pick axe, I dug straight down, assuming that I would eventually hit the gold I sought.
Unfortunately, it seems that the GPS program uses the reverse of the coordinates provided by the map program. So not only was I digging in the wrong area, but I ended up going down to the bedrock, straight through PROBABLY THE ONLY GAP FOR MILES IN ANY DIRECTION, and fell into the void to my death.
I no longer use Minutor and Minecraft GPS for prospecting.