There is a bug where if you switch users on your computer, the server does not kick you, but still thinks you are online. So as a result, tab lists don't show your name.

Also, logging into a server from another computer while logged in from another computer triggers this event.

Also, any achievements gotten in that time period are nullified, but others see you get it while you are in ghost mode.
How to recreate:
1. Log into any server.
2. switch users on your computer without closing minecraft.
3. go back and hit tab.

Another way:
1. Log in as usual.
2. Without logging out, go to another computer.
3. Log into the same server; you'll see a leaving message, but not joining message.
4. Hit tab.

I can still comment and play minecraft as usual though.
In the first picture, you can see I am in the chat as ClassicPork
In the second picture, I am holding tab, but my name does not show up.

Keywords i searched for prior to this:
TabList
Tab list
Player listing
switch user computer
switch user
player list switch user
14w20b