A few people have encountered this, mods included.
We believe the reason it happens is our bot detector is mis-firing occasionally.
Fortunately, after the Captcha is passed, it hasn't been blocking anyone afterwards.
@ThirdStringRFer I've had this happen, if you're using a vpn I believe it views some of the ip's used as possible spam/ robots and makes one do the captcha test, happens about once every 5 days.