Whenever I have to run the web installer to upgrade the database (for example), I have to disable plack and restart apache first because it caches the version number. If I leave plack enabled, the installer goes in an endless loop of saying the upgrade was successful, but never actually upgrading anything.
*** Bug 18972 has been marked as a duplicate of this bug. ***
I think I've run the web installer on Plack and it has upgraded but it does get stuck in a loop. Once I restart Plack, then everything is fine.
I haven't seen any reports of this for a long time. I assume it is fixed, please re-open if you disagree!