While recently upgrading a site to from Drupal 5.1 to 5.2 I kept on getting a "Site Already Installed" message and a redirection to install.php.

As this was a fairly active site I had little time to troubleshoot. So, I decided to do a database and file restore. After the restore I still had the same problems. How could this be? It should have been back to the same state it was before I started.

After a few minutes of frustration I cleared the cache of my proxy server. Everything is now back to normal. Except, I still have to try the upgrade again! Perhaps tomorrow!