Weird....Mine has a SmartXX, but I've chipped five more Xboxes in the past few months for friends and family with the Xenium Spice because I don't trust my soldering on other peoples' Xboxes and none of them have that issue. Err.....I guess that actually depends on how long is too long to have it unplugged. I know that my brother's was unplugged at least long enough to drive from Utah to California (10 hours or so) and I'm sure it was unplugged for longer than that cos they were moving so they had to pack it up, move, and then unpack. It was probably unplugged for days.
At least I'll know what to tell them if they ever do have a problem.