Sometimes it occurs that we loose the lock on a content while we are editing it (because of the unlock scheduler for example).
So the edit action is no more valid. A "Action is invalid error" is displayed to user when trying to save content.
This kind of error should be displayed as an understandable error for user and should explain the cause : "Unable to save content: the content is currently locked by user User 1" instead of "Action 2 is invalid"