We recently released a change to the way that Campaigns function, making them open ended. This means that subscribers will continue through the series when new actions are added to the end of it. This will happen even if the subscriber reached a point that was previously the final action in the campaign.
Because campaigns are open ended by default, subscribers will receive all messages in a campaign, regardless of whether the message was in the campaign at the time the subscriber began it.
For example, if a subscriber has received all three messages in a campaign, they will receive the next message you add to the campaign. This will happen no matter how long you wait before adding that next message to the campaign.
This also applies to wait actions. In the example above, if the subscriber received all three messages and then you added a new, one-week wait action after the third message, the subscribers would enter that wait period and would be required to wait one week before receiving the fourth message.
If, however, you add a message or wait action between messages one and two, then the action would not affect subscribers who have passed that point in the campaign already and the message would not be sent to those subscribers. The subscriber would not receive any new messages from the campaign until a message has been added after the messages the subscriber has already received.