From Joomla! Documentation

If you use memcached you may have a broken configuration screen. The best solution to this is to update to 2.5.11. If for some reason you cannot update to 2.5.11 you may apply the following hotfix.

Find the file libraries/joomla/cache/storage/memcached.php

At line 317 change Memcache to Memcached.

-    $memcached = new Memcache;
+    $memcached = new Memcached;