Zabbix actions with an invalid custom expression can be enabled by clicking the "status" columns toggle without validation.

This can be reproduced by doing the following:

• Add host group X
• Add host group Y
• Add host group Z
• Create action Action1 with custom expression "A or B or C" and condition "Host group = X,Y,Z"
• Remove host group Y
• Click Configure -> Actions menu options
• Select red "Disabled" to toggle to enabled

When actually editing the action manually and attempting a save it does correctly come up with 'Condition "B" used in formula "A or B or C" for action "JAMES" is not defined.' which means correct validation is being applied.

Now the side effects of being able to enable an action with invalid custom expressions was that this action ended up firing on any event causing 10000 incidents to be generated in our helpdesk system.

So I would like to think that when people toggle the status of an action validation is performed otherwise this can be an issue elsewhere.