Your not a true bitcoiner until you destroy some bitcoin. Or I should say you haven't experienced all bitcoin has to offer until you've lost access to a wallet.
When I first used blockchain on my phone I didn't write down my backup phrase and proceeded to lose about 54 cents worth. Thank goodness I didn't lose more. I should have send an even smaller test transaction, but whatever.
Does losing them on MtGox Count?
That was the only stupid Thing, I did with BTC so far ...
Well both my loss and getting Goxed are the result of stupid mistakes, however, my mistake was the result of a one time poor choice. Mistakes like sending coins to a non existent address or losing a password are the result of stupid errors made in a split second.
People who got goxed should have seen it coming a mile away. There were weeks if not months where it was very clear you should have gotten your coins out of there.
They aren't the same type of mistakes, but I guess the feeling is almost the same, and you learn similar lessons, so I will say, yes it counts.