Below is a screenshot showing a page layout bug. The trig expression displayed in the bottom of the preview window should not be there - it's part of the question - not part of my answer. Other instances are much worse, with the preview being totally overwritten and unreadable. Reproducible in IE8 on XP SP3.
EDIT This bug still exists - see the 2nd screenshot below. I was able to reproduce it again while editing my answer here. It appears to depend crucially on contextual information so may be difficult to reproduce.