The UK has the notion of a "strict liability" law. If you are caught with a picture of child abuse - you're guilty of a crime. It doesn't matter if it was sent to you unsolicited, or misaddressed. Possession is the crime and there are no mitigating circumstances.
On that cheery note, let's consider Twitter's new image embedding functionality. If your friends post a photo onto Twitter, you will see it in your timeline automatically. No need to click on anything. (As an aside, that's a feature Dabr and other Twitter apps have had for years).
Imagine, for a moment, if it had contained a pornographic photo. Everyone who followed Britney would automatically be exposed to explicit images. An annoyance for some, an employment disciplinary matter for others - and for some, a legal issue.
Suppose a some hacker takes over the account of a well followed Twitter user. They then upload images which are illegal in the UK - and possibly other countries. Now, everyone who follows them, has that image in their web browser. It's stored in their cache, it may even have been auto downloaded onto their phone.
That's what happened to me last week.
A friend posted a photo of the new Beady Eye single.
True, it's not the most explicit image in the world, but it's not the sort of thing you want on your screen at work. Nor do you want it in your browser's cache should someone decide you need investigating.
All of a sudden, all of this user's followers have an (unwanted?) image on their computer. In this case, half a breast is unlikely to cause much offence - but this could easily have been the controversial Virgin Killer album cover.
This thrusting of potentially illegal images can also happen by accident - a Twitpic error caused the BBC's technology correspondent to appear to share a photo of a woman "in a pose that can only be described as extremely post-watershed."
So, what does this mean for Twitter and for strict liability laws? It's almost impossible for a computer to automatically detect whether an image is offensive or illegal. Humans aren't much better - and there's a massive cost for pre-emptive moderation.
A wave of spam on social media could be enough to see you convicted on some very nasty charges.