If you try to modify a style attribute on say,
* HTML * BODY * DIV * DIV.inner_page * DIV.page_header * DIV.navigation_bar * UL.navigation * LI * SPAN.link
by double-clicking the color attribute of the last rule (body, th, td, a, div, span, label, input, textarea, select) and then going into the rule field and trying to exit again, you can't actually ever click out of the new rule field or even make a change to fix the loop condition after you accidentally try to click out. It forever pops a dialog that says 'Invalid selector text!'. You basically have to kill the browser to end this loop.
For best results, use the Firefox browser..