Funny thing happened today when I tried to export / import a site from one site collection into another one. It asked me to close the Summary Links web part, which I did. I tried to add it manually (importing a .webpart file exported from the source site), and it gave me the following error:

Unable to add selected web part(s). Could not load XSL file. The system cannot find the file specified.(Exception from HRESULT: 0x80070002).

SOLUTION:

This error is usually raised when you try to add a publishing-related web part (such as Summary Links) into a non-publishing site. You have to either add it into a site built with a publishing template (Collaboration Portal or Publishing Portal), or to activate the publishing features manually:

• Activate Office SharePoint Server Publishing Infrastructure feature in site collection features, in Site Collection Administration section of Site Settings page.
• Activate Office SharePoint Server Publishing feature in destination site features, located on Site Administration section of Site Settings page.