When I was new to Minecraft I thought, "Oh, hey, this is the game they're talking about. This is my first time playing. It's gotta be exciting." So I watched my brother play and he made a pool of TNTs using Too Many Items and hid in an obsidian house.
So I borrowed the same character in the same world and tried it out. I was new to Too Many Items and I didn't know what to use to mine the obsidian. I made a closed house of obsidian and went inside (without torches or anything, full floor, wall, and roof obsidian). Then I became like, "Okay... how do I get outta here?"
This is quite common I believe... I was exploring a cave with lots of items, thinking "these caves won't do any harm" and going inside. Then, I meet lava and some other mobs. I die and lose my items. Nowadays, I only bring a stock of pickaxes and torches and move on.
When I first found out about The Nether, I did all I could to travel to it as fast as possible. Once I got there for my first time, I immediately started rushing in and attacked a Zombie Pigmen because I thought it was gonna attack me first. All remaining Z. Pigmen killed me, which could've been avoided if I did nothing at all.
Being so excited over SinglePlayer Commands that I /explode 100'd all of my original worlds. To this day I am undergoing mental therapy to avoid permanent phsycological damage because of this action...