I'll start with mine.. so wasn't still quite sure how bitcoin worked in relation to difficulty, hashing, etc, so I bought a second HD 5770 when price hit $30 thinking I could get a 6990 with not much more work after that....
and then the crash, and then the difficulty hit where now my 5770s are earning "half" of what they were when coins hit $30 ea.
Had I not been a n00b, I would have sucked up and got a 6990 instead.
So now I'm still less than 10btc, and waiting to build back up *sigh*
What n00b mistakes did/have you made???