Glooble found out that if you /r to reply to a player who /msg'd you when they aren't logged in, the message will still go to them. Not a huge problem, but still notable. I then tried /msg'ing a player not logged on, but it didn't work.
/r means /revive You revive the player who logged out for 0.000000009 seconds but you can see they logged in and out :3