Here's a weird one ... but I'm sure there's an easy answer.

On a couple of my sites, I seem to have created a rectangle on the page background. It's white, has a yellow border, and it's fixed in place. It's below the actual site, but I can see it flash by at times when I'm working on the design. I can hide everything else and see it, but cannot move it, resize it, or remove it. Any ideas?

Thanks in advance! This forum is super!