Today, 21st February, at 18:49 a player called "chinichi" popped up in my server's log. The server's on online mode, so no non-premium players could join. How comes chini came on my server?

To chini: i know that's sad, i'm not talanted at making spawns. That's the best i could do. Any help would come in handy though.

and why is a post for that needed?
he can just join any server he wants unless you ban him of course

Uh, what do you mean...

Premium, when you use online mode, means that only players who have logged in can join.

Chini brought the game, and logged in when he started up the minecraft client. You know, how you have to press the "login" button when you start up minecraft? Once you log in, you can join servers that have online-mode in server.properties set to true...

He joined your server because it was advertised on Aprilon.
See this http://www.aprilon.net/forum/viewtopic.php?f=12&t=2183

Because I feel honoured he joined me

Edit: Nevermind