Not everyone reads the upgrade notes. There is a problem of lack free disk space/big database size due to the disabled housekeeper after the upgrade (especially from 2.0 to 2.4).

I believe, we should add a warning like "Housekeeper is disabled by default after upgrading to 2.2. The desired housekeeper functionality should be checked and enabled manually if it is necessary." to the upgrade procedures also:
https://www.zabbix.com/documentation/2.2/manual/installation/upgrade
https://www.zabbix.com/documentation/2.4/manual/installation/upgrade