There is somewhat recent research (PDF) which has found there to be at least links to child pornography on the bitcoin blockchain with also one one explicity file on the blockchain which may be classified as actual child pornography.
Now, assume there is actually a clear and indisputable case of child pornography on the bitcoin blockchain. This means that everyone who a) mines bitcoins and b) verifies the blockchain themselves have a copy of this file on their machine.
This raised (at least for me) the interesting question:
Is there any country / jurisdiction in which it is not illegal to own child pornography (on your personal computer)?