There is currently a nice craze going around where people are linking usernname/password popups to images.  People then use these images in their sigs or as avatars so whenever you load a forum thread containing a post by them, you get the pop up.

It rips the web page id from the forum it's on and looks like a pretty basic login auth and lots of people have fallen for it.

At the moment they are hitting the Minecraft forums pretty hard and have 'stolen' several minecraft accounts over the last week thanks to this, which the mods there are trying to get a hold on.

This is just a heads up.  This forum will never ask you to login via a popup it's all done on the main page and if you regularly use other forums be very wary of any similar kind of login request.  If you play Minecraft and think you may have fallen for this, change your password asap.

I use noscript and adblock plus and I never saw the popup so I'm hoping most people won't even know there was an issue like me.