We had to set up a MacOS (OSX) for testing iPhone app-development, but it was really awful to realize that mouse speed and input language, or applications that run as services (teamviewer) can be changed by any user that uses the system. We connect to the system by VNC, but one of our collegues is next to the device and uses wired connection, and he gets strange effects like seeing an user working on his desktop and so on. This only happens if the VNC-user logs in before a non-VNC-user ever logged in to the system. Is MacOS (OSX) a multi-user operating-system?
OS X in itself is a multi-tasking, multi-user OS, but VNC is desktop control software, so software that controls 1 desktop (unless set up in such a way that multiple virtual desktops can be controlled remotely).
So what you're seeing is standard behaviour of the software you're using, not of the underlying OS...
Since you do not specify if you are using OS X Server or not, let me presume not and suggest that you might be violating the EULA:
2H. Remote Desktop Connections. Subject to the terms and conditions of this License, when remotely connecting from another computer or electronic device (each a “Device”) to an Apple- branded computer that is running the Apple Software (for purposes of this Section, such Apple- branded computer is referred to as the “Home Mac”), whether through the Screen Sharing feature or through any other means:
(i) only one (1) Device may remotely connect at any one time, whether directly or indirectly, to control the graphical desktop session of the Apple Software that is running and being displayed on the Home Mac; and
(ii) a reasonable number of Devices may remotely connect at the same time for the sole purpose of simultaneously observing the same graphical desktop session of the Apple Software that is running and being displayed on the Home Mac, as long as they do not control the Apple Software in any way; but
(iii) only one (1) Apple-branded Device may remotely connect at any one time, whether directly or indirectly, to control a separate graphical desktop session of the Apple Software that is different from the one running and being displayed on the Home Mac, and such connection may only be made through the Screen Sharing feature of the Apple Software.
Except as expressly permitted in this Section 2H, or except as otherwise licensed by Apple, you agree not to use the Apple Software, or any of its functionality, in connection with service bureau, time-sharing, terminal sharing or other similar types of services, whether such services are being provided within your own organization or to third parties.
From here .
So that boils down to the fact that legally you can connect multiple devices to view the same session, but only connect one other Apple device to control a separate session.
After a *very *quick glance I don't see the same restrictions under OS X Server.