Hi Angus Cheung,
Thank you for your feedback.
We will do our best to address this issue in the next version of RadScheduler for Winforms. Currently, there is no workaround for it in the Q3 2009 SP1 version. However, we have addressed a similar issue regarding drawing of invalid rectangles in our suite and the fix is available in the latest internal build. Please download it and give it a try.
I have updated your Telerik points for your feedback. Please contact us again if you need more information.
All the best,
the Telerik team