Here's an odd one...

[sabrage]

When an HDMI cable is connected to my video card (an HD6850) the LEDs on my case's fans light up... Regardless of whether the power is on. Even when the PSU is off or completely unplugged, the lights still glow. What the hell is causing this? The HDMI cable is attached to a switch, which is then attached to my monitor via an HDMI>DVI cable.

[Mistabashi]

That's actually rather worrying. I'm guessing it's a powered switch box you're using? Sounds like it's feeding-back current through your motherboard, which would be rather a problem. Can you try without the switch box connected?

[Heliocentric]

HDMI is leaking energy from your monitor to your pc, the fan is feeding off energy going the wrong way back down your power cables.

Its funny when you think about it. I bet you are powering your fan with an adapter on the power supply main feed rather than via the motherboard, its a shame it isn't spinning your fan. A fan spinning after shutdown for a minute can really improve the lifespan of your pc.^_^

[sabrage]

The switch isn't powered, and the lights obviously don't turn on when the other end of the HDMI isn't connected to it... The fans in question aren't connected to the motherboard at all, like Helio speculated they're connected to molex adapters. It seems really odd to me that the power is being routed through my video card back to my PSU, though... Yet another reason I regret AMD. I'll have to give it a shot connecting the video card straight to the monitor; it's a little hard to access with my setup.

If they were only staying on for a minute, it wouldn't be a problem because I could easily just forget about it. I don't power my computer down all too often, but when I do I want it to stay down.

[Heliocentric]

The switch isn't powered, and the lights obviously don't turn on when the other end of the HDMI isn't connected to it... The fans in question aren't connected to the motherboard at all, like Helio speculated they're connected to molex adapters. It seems really odd to me that the power is being routed through my video card back to my PSU, though... Yet another reason I regret AMD. I'll have to give it a shot connecting the video card straight to the monitor; it's a little hard to access with my setup.

If they were only staying on for a minute, it wouldn't be a problem because I could easily just forget about it. I don't power my computer down all too often, but when I do I want it to stay down.

DC circuits are super simple, its not really AMD's fault, its more to do with your power supply not being solid capacitor I think. I'm sure this happens in many PC's but they don't have an indicator light, its just a quirk of HDMI, your PSU and your LED fan all together.

I have one, I have a USB powered speaker with an LED which remain lit up unless I unplug my printer!(also usb)

[Feldspar]

I have one, I have a USB powered speaker with an LED which remain lit up unless I unplug my printer!(also usb)

That's not powered by electricity, it's powered by the love your speaker has for your printer. Usually computer peripherals are doomed to live lonely, loveless lives, but sometimes there is a special spark, two electronic boxes that share a USB connection of the heart, that's what keeps the LED burning.

[mrwout]

That's not powered by electricity, it's powered by the love your speaker has for your printer. Usually computer peripherals are doomed to live lonely, loveless lives, but sometimes there is a special spark, two electronic boxes that share a USB connection of the heart, that's what keeps the LED burning.

I suddenly felt the urge to cuddle my printer...

(sadly I've nothing really on-topic to say :( )