If you're a WordPress user and are in the process of an upgrade from PHP4 to PHP5, you might be having a few issues. One problem can be cause by the wp-cache component and this recent post from Shantanu Goel.
The issues range from weird page layouts, to some controls not working, to some errors popping up here and there, and probably your blog not even displaying. This occurs because even though WordPress is PHP5 compatible, some of the plugins you are using might not be.
In his case, it was the wp-cache plugin that was causing the problems. Permissions weren't right to allow it to do its job. This resulted in problems rendering content and with the site acting as it normally would. His solution involved disabling and reenabling the plugin after deleting the cache and lock file the plugin uses.