The FireWire Drive That Would Not Die!™

So yesterday afternoon, my boss hooks up an external LaCie FireWire hard drive to one of our G5s. We’re working on a project for the Apple retail stores, and we need to take some screenshots. My boss was going to boot the G5 off of the FireWire drive, then use Snapz Pro to take said screenshots.
After attaching the drive, we saw it pop up on the desktop. My boss went in to System Preferences –> Startup Disk, and selected the FireWire drive. Hit restart.
And the G5 reboots, but doesn’t start up from the FireWire drive. Hmmm. We know the image on the FireWire drive is good, and the G5 can boot off of it’s alternate internal partition, where the same boot image is loaded (but doesn’t have Snapz Pro or any of the other tools my boss was planning on using). Certainly an oddity.
Boss powers down the G5, but the FireWire drive stays on! Is it still sucking power from the G5, even though said G5 isn’t powered on? The conversation went something like this:
Boss: Can you unplug the G5 for me?
Chris moves behind the rack, steps up on the stool to get to said G5; pulls power cord.
Chris: Power cord is pulled.
Boss: Dude, the drive is still on!
Chris: No way!
Boss: Come around here and please tell me I’m not hallucinating! (Always a strong possibility, as said boss used to be heavily involved in the music biz. Just kidding, boss!)
Chris moves around to front of rack, and verifies that the FireWire drive is still powered on.
Boss: You’re sure you pulled the power on the right G5?
Chris: Positive.
Boss: Can you please verify? This is just too f&#@*$g weird.
Chris moves back behind rack and verifies that the correct G5 is unplugged.
Chris: G5 in slot #19 is unplugged. Ah! I think I see what the problem is.
Care to venture a guess before we move on?

What had happened was that the G5 in question had been hooked up to another G5 via a FireWire 800 cable. We booted the G5 in question in FireWire Target Disk Mode initially, to partition its drive and to blow the restore image on to it, since it was faster to do it this way than off a FireWire 400 hard drive.
What we failed to account for was that the G5 in question was still plugged in to the builder G5! That was how the external LaCie drive was still getting power, despite the fact that the G5 it was connected to was powered off and unplugged from AC.
Ah, the wonders of technology…