Facebook security
I was experimenting with a little known feature in Facebook, "Download Your Information" which will actually supposedly give you a copy of everything that is 'yours' on Facebook.
The definition of what is 'yours' is fairly tricky of course: is what you posted on someone else's wall 'yours' or 'theirs'? And so on.
But what interested me was how they made double and triple sure that in fact it was me who was downloading my information.
I had to supply my own password again: ok that makes sense.
But then for extra extra security I was shown a bunch of wall photos of people who are my 'friends' and asked to identify them from a multiple choice set of friends. This is harder than you think: not every friend is such a good friend. And not all the wall photos are recognizable. They might be childhood photos, or out of focus group shots at a party or whatever.
But really quite a smart way to make sure that the downloaded content does not fall into the wrong hands.