Thank you for contacting us and for the issue reported.
Most probably, this is a RibbonBarGroupSeparator. I did some tests, since you are the first to report such an issue and cannot reproduce it with my test project.
Is it possible to send us the problematic project where the issue can be reproduced? This will allow me to investigate the case in details and provide you with a helpful response.
Please note that you have to open a new support ticket in order to be able to attach files.
the Telerik team