No worries, really a minor thing but is happening for me on both chrome latest(43.0.2357.130 64bit) and firefox developer 40.0a2 on OS X 10.10.4.
Click log in, pops up a window where I either login or it says "use this account you're logged into?" and then drops me back at the forums with a signup modal(and when I close that, still logged out).
Meteor has been great for me though, you guys are building(have built) a really cool thing.
I'm sorry! I haven't heard any other reports of an issue like this; what browser/system are you on?