Not so much as a client, as it was my own mother. For months she had been complaining to me about how her computer kept crashing and losing all of it's data. She would take it into a local repair specialist, as I live about an hour and a half away, and he would reformat the drives and rebuild the bios, then everything was fine for another month or so when it would happen again. This cycle repeated itself about 3 times, and I had been chalking it down to the fact that she owned a 5 year old Packard Bell. So I finally make it home from college for a weekend, and I'm sitting at her computer desk talking about various things as my hand absentmindedly probed beneath the case (It was a horizonatal casing). When lo and behold I find something and pull it out... it's a HUGE flat refrigerator magnet. Must have been 6x8 inches, sitting right underneath her case.
Me: "Uh, Mom, how long as this been here?"
Mom: "Oh, I don't know, one of the kids must have put it there a while ago."
Me: "Well, just so you know, magnets wreak havoc with computers, this is probably why you keep crashing."