Not exactly a bug, but more of an overlook.

When someone uses /w to send a private message to another player, it generates command feedback containing the private message then broadcasts it to every operator in the current world. Again, not a bug, but it could render /w useless in worlds where everybody's an operator.

Another example where it could be an issue is when you're on a realm, you have a secret base, and then you /w your friend the co-ordinates, but an operator is online, and the operator sees the message, then decides to troll by posting the co-ordinates publically in the chat where everybody can see it.