I was working with ItemStyle.xsl to customize the look of a Content Query Web Part and right about lunch time, I made a breaking change to the xsl. I didn’t realize it, but this had far reaching effects throughout the site collection. I went off to lunch and upon my return, noticed this message appearing in a bunch of places:
Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Windows SharePoint Services-compatible HTML editor such as Microsoft Office SharePoint Designer. If the problem persists, contact your Web server administrator.
I blamed the client (not realizing as yet that it was my fault at this point) but eventually noticed that visual studio intellisense was warning me that I had malformed XSL. I corrected it and everything started working.
Be darned careful when working with ItemStyle.xsl (and any of the global XSL files) — breaking them affects many artifacts in the site collection.