well of course it's a stupid brainwallet. it's the empty string!
I mean it may well have been generated by someone deliberately hashing an empty string, just as all brainwallets are created by the user deliberately choosing a particular string to hash, as opposed to some flawed software hashing an empty string while the user believed it was doing much more than that.
what if you filled a bag with dice and blindly pick one die at a time and put it on the table and then look at the number on top. that eliminates any bias that is in the dice.
It doesn't eliminate any bias at all. It simply mixes the bias of each individual die among the bias of all the dice, and you hope that doing so is enough to maintain the security of your resulting entropy. And if you go out and buy a set of 100 dice to do this with, how do you know that every single dice in that set hasn't been subjected to the exact same manufacturing defect and therefore has the exact same bias as every other dice?
but again, i challenge anyone to show me a story where someone used dice to generate their bitcoin private key and then later said they got hacked. if they got hacked it's because of something else rather than a bad private key.
In addition to BlackHatCoiner's response, it is often impossible to pinpoint exactly how a seed phrase or private key was compromised, so asking for such an example is meaningless.