You can see who is OP by typing /deop and pressing TAB even if the player doesn't have permission to use /deop

In multiplayer servers, if you type

/deop

and then press TAB, you'll be able to see who is OP, even if the player doesn't have permission to use /deop.

This is very annoying and is especially bad if your server is on offline mode. (My private video-recording server is on offline mode.)