VGA connector delivers 5V DC to mainboard, powers fans when shut off

[Neon Ape]

So I'm building a pc for my housemates little brother.

Today the powersupply arrived and i got everything running.
While troubleshooting a possibly dead x16 slot i ran into something weird.

When i shut down the computer and turned off the powersupply the fans kept spinning. I unplug the powersupply, they continued running. I switch the fancontroller to 12V, they stop. Turn it back to 5V, they start up again.
I'm a bit baffled at this point and go over everything. The only other thing connected to the pc is the vga cable running to a monitor. I unplug that, fans stop spinning, plug it back in, they start up again.
After a bit of wiki reading i find out there is a 5V pin in a vga connector, but the spec says it's there to power some chip in the monitor.
I guess i found out it goes both ways and my mainboard just passes through 5V from the vga to the entire system (the fans run off a controller plugged into SATA power).
 

Has anyone encountered this before?

Is there any way to fix it or should i just not care because the system will be running dedicated graphics anyway and the problem only appears when plugged into the onboard graphics.

Mainboard is some Acer OEM board with an i5 650, PSU is a Chieftec APS-450S.

[emosun]

No , that's one really weird problem. The vga connector ( I suppose the monitor itself) shouldn't be sending power to the machine.

use a different monitor. 

[Nevexo]

Your monitor should NEVER feed power back into the device unless it's some special HDMI thing. Use a different motherboard that could damage your motherboard.