The first two times it happened were far between, but it happened three times while I was typing my next post ... always after I hit my space bar.
I can't get it to happen under similar conditions, so if I had to hazard a guess, it's a glitch in your software (as opposed to FTM's), although I'm hard pressed to understand how.
How about clearing your FTM cookies, as well as your cache?
No idea whether those arenas might contribute to the issue, but it couldn't hurt to give it a shot.