Consider the simplest cycle, a node that points to itself. To create it, you must include the content hash of the document in the document itself. To do that,

Consider the simplest cycle, a node that points to itself. To create it, you must include the content hash of the document in the document itself. To do that, you need to generate a collision. You enter a random content hash in the document, then keep modifying the document and hashing it untill it's hash matches the one you included. It's not impossible, but depending on the hash function, it may take you a very long while to find it.