Because of a slightly flakey connection, I think I was just prompted to prove I am a human when trying to post an answer.
However, although the dialog box popped up and the rest of the page was masked with an overlay, instead of the Flights of the Concords screenshot and the captcha prompt, the dialog box consisted only of the immortal text
I think this means Stack Overflow is on to me and knows I am not human, and is trying to talk to me in Robot language, a silent little language only Robots can understand*.
Sorry, no screenshot. I was a little flustered at being called out like that.
*With apologies to Tomten
Update: Martin Smith provided one:
and I reproduced it the same way (paste into the sandbox and TAB then ENTER to post something fast to see the error:
I'm using Chrome 31 on OS X.