First of all, sorry about that. The Pagespeed team put a lot of work into validating the rewriters against a large corpus of websites and disable filters that cause problems as soon as we can, but sometimes things slip through.
If it's still breaking your site, please raise a support ticket. If you can, including the following information will make it much easier to diagnose:
Try appending ?ModPagespeed=off to the URL. This de-activates PageSpeed. If the site is still broken, it is not a rewrite or HTML parsing problem. It might be a configuration clash.
If that fixed the site, try appending ?ModPagespeed=on&ModPagespeedFilters= to the URL. This turns on PageSpeed, but no filters. If the site is broken now, it is an HTML parsing problem. Please let us know.
If the site still worked, try appending ?ModPagespeed=on&ModPagespeedFilters=foo for various filters "foo". For example try:
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=extend_cache
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=combine_css
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=inline_css
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=inline_javascript
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=insert_image_dimensions
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=rewrite_images
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=rewrite_css
http://www.modpagespeed.com/?ModPagespeed=on&ModPagespeedFilters=rewrite_javascript
You may have to reload them a few times over several seconds to make sure they have had time to load sub-resources into cache and rewrite them. If one of these breaks your site, you now know which filter is at fault. Please let us know. You can disable that filter by adding a line to your .htaccess file:
ModPagespeedDisableFilters foo
Article based on Pagespeed "Frequently Asked Questions"
