This is a div with a background image (blue). Both div and the image are 600px width and 400px height.

The body also have an background image and it's 2000px width and 400px height. That image is white, but it contains a black rectangle in the middle. The rectangle is exactly 600px width and 400px height and you can't see it, because the div's background is covering it.*

Open this page with the Opera Browser and make sure you have the scroll bars enabled. (Settings → Preferences → Advanced → Browsing → Show scroll bars)

Click here to toggle the div's margin-bottom from 0px to 2000px or from 2000px to 0px.

The scroll bars should appear. But the 1px black line on the right side of the div (part of the black rectangle) shouldn't be visible.* Try this on another browser. It happens only in Opera Browser. And it seems to be happening only when the background's image width is larger than the body width.

*[UPDATE]: On 11.60 and 12.00 alpha (build 1191) the 1px black line is visible when the scrollbars are not displayed. On 12.00 alpha (build 1211) (and previous releases, like e.g. 11.51) the black line is visible when the srollbars are displayed.

PS. Sorry for my bad english... :P