IF (big if) somebody managed to insert a noticable amount of real child porn in such a way that a simple one line command could extract it, waits for it to be really deep in the chain before announcing it to the public, and if this happened before the filtering and pruning mechanisms are implemented - I'd imagine we could have a problem. Or maybe embed ROT13s of phrases that are highly offensive and inflaming to lots of religious people and would likely prevent them from ever accepting bitcoin as long as they remain in the chain - say "Allah is evil and not a true god" or some such. Again in such a way that anybody could extract them with a simple command.

In such a case what could be done after the fact? Could they still be trimmed out of the chain without invaldiating the block links and the whole transaction history after they were included? Or is that impossible at this point?