Just wait until somebody encodes kiddie porn into the chain - it would stay there forever.
It is impossible to completely prevent that kind of abuse unfortunately.
For example, someone could encode information in the decimal figures of amount sent, or in a "vanity plate" Bitcoin address.
Probably not practical for jpeg or mpeg files, but for other low-bandwidth "illegal data" such as Blu-Ray keys, this is bound to happen sooner or later.
Even "low bandwidth kiddie porn" could create big problems for us one day. All over the world, legislation is becoming more draconian every year and in some countries even the "wrong" kind of fiction and poetry is now considered kiddie porn by the courts and thus illegal.
