We have had an old version w3tc installed for over a year. Today we decided to update to the latest version of the plugin. After a minute or so after the plugin got activated all CPU got consumed and the number of apache threads spinned off like crazy. We deactivated the plugin and removed the old configuration and reactivated the plugin. The problem was still there, even without having any caching activated in the plugin.
We're running the latest version of WP and Apache/2.2.14 (Ubuntu)