Deleting a template that is part of an action condition won't remove that condition and displays 'Host template = Unknown' instead.
This behavior is not really bad, because the action should not be changed silently what could lead to unwanted action operations.

I noticed this issue by the small information 'No host with hostid "123456"' in the bottom line and was forced to query the database to identify the corresponding action.
It would be nice if either the information additionally directs to the affected action or deleting a template that is member of an action is prohibited

Haven't tested if this behavior exists for other condition types too.