Mixed RenderModes - when the page contains multiple controls of the same type set with different render modes. In such cases, the stylesheets for both render modes are loaded on the page, where one would override the rules of the other and this results in a bad appearance. Check out the RenderModes for more information.
Also, it would be very helpful for us if you could provide with more information about the issue, grid configuration. Once we can replicate the problem, we'll be able to point out the cause and provide with a solution to fix it.
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