Well... Where to start?....
I guess I would start with I already knew alot of things about the game when I first played it.
But I did stupid things like the following:
Tried to mine Bedrock
Jumped off a high cliff (Darn you suicide,why did you have to exist?!)
Went into caves at night being stupid
Let mostly everyone in a village die (I grieved for a few seconds then took their crops)
Went into creative mode so I could get enough obsidian to make a nether portal (I never returned)
Let my wolf jump into an End Portal (He never returned)
Ate an spider eye (NEVER AGAIN!!)
Tried to face an Ender man head on (NEVER AGAIN!!!... too?)
I have tons more of noobish blunders and mistakes
but those will most likely not be told another time.