In the process of trying at install plugins on a new blog, I kept getting this very user unfriendly message
Incompatible archive PCLZIP_ERR_BAD_FORMAT (-10) : Invalid archive structure
I was trying install various plug-ins into the default installation of Wordpress 2.8 from my hosting company.
The fix was fairly simple. You need to replace the version of ../wp-includes/http.php [...]