Such appearance of RadDock is usually encountered on a zoomed page, opened in a IE browser. When zooming under Internet Explorer, the background image of the RadDock doesn't scale and ends up showing the other sprite images. This is a browser specific behavior and it could be reproduced with any elements that are using background images. You can check if this is the problem by pressing Ctrl + 0
when the page is opened in Internet Explorer, as this will set the default zoom.
I hope this helps.
the Telerik team
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