On a desktop computer, I often get ads that take the "focus" away from the comment window. So when I'm typing, a comment... suddenly the keystrokes aren't being registered in the comment window.
Once while typing, I was suddenly at a McAfee website telling me to load some virus protection... I don't know if the ad that took the focus then registered my keystrokes as a "click"? I don't recall a McAfee ad, but I wasn't looking at them while I was typing. I hit the back arrow on the browser and went back to BF, so I definitely left this site.
Ad refreshing that take away the focus seems to happen enough times that it isn't ad specific (or many ads to it?).