Just researching another possible threat.
What if an individual who is angry at Bitcoin for whatever reason tries to insert some illegal content into blockchain?
There are a lot of content types that are illegal in one or many jurisdictions, for example Charlie Hebdo cartoons in Pakistan, suicide instructions in Russia, Sony PS3 signing key in the US, etc. This person would inject illegal content into OP_RETURN outputs, maybe spreading large content over several 40 bytes OP_RETURN outputs, and this content would be stored forever. He would inject it in the most open and self evident form possible, no encryption, no steganography, no fancy encoding. The content would be stored and distributed by full node operators. Their activity might not be automatically illegal while they store and distribute this content **unknowingly**. In the pre-blockchain world, it is common practice that if you run a web site that unknowingly stores and distributes some copyright infringing or otherwise illegal content and you receive a take down notice, you comply and you are ok after that. But you can't remove anything from blockchain without destroying its integrity. So a full node operator can only comply by ceasing operations, and if he doesn't comply after receiving a take down notice, he would **knowingly** store and distribute illegal content which places him in bad terms with the law.
This thread
https://bitcointalksearch.org/topic/so-what-happens-if-i-violate-the-block-chain-128171 discusses pruning the offending transactions, which is not a simple thing as it would require changes to verification procedures. I understand it was never really necessary and never implemented. Even if it were implemented, it would create a mess if some nodes pruned a transaction while others did not. Also, the inputs of fully pruned transactions could be double spent.
As Bitcoin grows and becomes a business for many, the risks of illegal content become more important.
Any other ideas how to mitigate these risks (if they even exist)?