Bitcoin

So, the bitcoin ledger is both immutable (it can only be extended) and available to everyone:

• if it's not immutable then something is horribly wrong with the underlying cryptography, and bitcoin also fails at one of its main purposes;
• if it's not available to anyone then only the privileged elite to whom it is available can check and make transactions, which means they control bitcoin and again bitcoin fails at one of its main purposes.

And the bitcoin ledger consists of bits, which can encode transactions ... or anything else. Such as, you know, classified information, defamatory material, illegal pornography.

Oops.