Thank you for the screenshot and code snippet.
The observed visual glitch is expected in this case and it is caused by browser behavior and the browser's implementation of the CSS standards.
This is what happens actually:
RadGrid has a default width of "100%", which is applied to the control's wrapper, which is a <div> element. The <table> element inside has also a default width of "100%", however, tables can expand horizontally to enclose their content, while <div>s do not expand horizontally. As a result, the <table> overflows out of the <div>.
Possible workarounds are:
1) Place RadGrid in a wider table cell.
2) Set an adequate width for RadGrid in pixels.
3) Remove RadGrid's border like this:
the Telerik team