I don't thing it's such an edge case. I'd be willing to bet that at least 90% of people on HN have witnessed child pornography at least once on the Internet. That's a lot of criminals. Spend a few minutes on 4chan's /b/ and you will probably see some.
But you could be. Universal crimes that are only prosecuted when the police want to get you are a large part of the problem. Everyone is guilty of something - see Three Felonies a Day.
In that case you are criminalizing something other than the thing you actually want to stop. It's not walking across the street that's the problem, it's running into traffic. In this relatively low stakes problem people still run into the problem of not knowing the unwritten law. For instance people from cities where jaywalking is never prosecuted go to somewhere where it is aggressively prosecuted and get shafted.
