One of the most common problems related to IE6's more correct interpretation of the rules results in pages with fonts that are prodigiously larger or smaller than intended. Relative font-size (em or % units) should be calculated relative to the containing element.
So if you set:
body, p { font-size: 200%; }
the text in <p> will be *four times* the default font size. This may not be what you want!
IE6 now correctly interprets the CSS "width" property and correctly calculates relative units. This could have a visible impact on previously well-ordered layout.