Seriously, I see this all the time. I have so many computers, one of them is bound to be doing this at any given time. It's not really dangerous, just annoying, and I have a possible fix. I use chrome, and I get this in chrome. It does not appear to be present in Chrome Canary, which is basically just the dev version. You can download and use it even if you're not a dev, and it's not as horribly unstable as you might believe, so give it a shot if this is an unbearable problem.