I recently ran into a very vague problem. It's happened now 3 times that (seemingly random) pages on one website redict to a different website. I have multiple sites running on the same server. Using .htaccess and the httpd config file, Magento determines which site to show. It's always worked fine, but last week sometimes randomly pages from one site would load the homepage of another site. After clearing cache, the problem seemed to go away. But a few days later it happened again.
Also, sometimes when in the Magento backend, when this is occurring, the Magento Install page will show. Afaik I didn't do any changes that should cause this behavior. Last I was working on datafeeds. In the error log, the only thing I could see was Zend Guard Loader was not the right version (and it has since been updated) but in the weekend this randomness occurred again.