I had once wrote a wrong message (nothing worth of notice) to my wife, so I just deleted it (and deleted for her, too) instead of just messaging her again saying that the previous message was wrong because blah blah blah. She called me and said that a strange thing happened: she had a notification with a message from me (and she could read the first 4 or 5 words of it) but then the message was nowhere to be found...
So I explained the "delete" function, and that the message was wrong so I just deleted it.
But still she was able to read the notification AFTER I deleted the message, so, basically, you cannot unsend what you have sent.
Biting the hand that feeds IT
