I have created a Roksprocket "List Layout" module and configured the Preview Length to display 30 characters before it inserts a 'Read More' link.
I can't include a screenshot because the site is on a dedicated Intranet.
The issue occurs when the 'Read More' breaks up a bolded text string. The result is that the remaining text in the module becomes bold and sometimes displaces an adjacent module. Apparently, the 'Read more' does not include an ending </b> or </strong> tag.
If you set "strip html" to "yes" that would fix the issue. But if you actually want the html then you have to ensure that the preview length will include a balanced html tag set (ie for every opening tag there is also a closing tag).
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