I have found the trigger a little free space on a server was monitored as OK after reaching 0% free space. Since the trigger is configured to alert when the free space in% is less than 10%.

Zabbix server:
Zabbix Server 1.8.8
Red Hat Enterprise Linux Server release 5.3 (Tikanga)
Mysql 5.1.53-community-log

Monitored server:
Zabbix Agent (daemon) v1.8.8 (revision 21965) (28 September 2011)
Red Hat Enterprise Linux ES release 4 (Nahant Update 5)

Attached is the image of the item, the trigger, the recent data, triggers and events.