I think this happens when the css file isn't loaded properly in the browser. If it happens once in your thousands of pageviews then I wouldn't worry as it is just a problem somewhere between all the stuff from the BYC servers to your eyeballs.
If it happens once every 100 or less pageviews then you've got a problem with your system (connection, browser, etc.). I can say I know it is isolated because if it was happening more frequently to all visitors, I'd have 10000000 emails and pms.
Of course, anything is possible, so if anybody is having this as a consistently frequent problem please keep us posted.
It is only happening to me on the trivia contest. Probably because there are so many people looking there and I am in a hurry. I got it 3 times, but if you refresh it gets back to normal.