Steps to reproduce

1. Insert two body macros (e.g. expand)
2. Attempt to insert a horizontal rule via the toolbar

The rule will not appear. If the user continues to type, the cursor will jump to the top of the page.
This behaviour has been reproduced in Safari.

The rule will not be inserted in IE9 or Firefox. However, the cursor jumping will not occur in these browsers.
Also note that the cursor jumping does not occur in the ERT theme.

This issue does not occur in IE8 (I am as surprised as you).