PC World does this every few years or so. They've used a faulty IDE cable, and, uh, I can't find any of the other articles.

There several schools of thought on this sort of thing, IMO. If they're trying to determine who's simply cheating you, do something simple and obvious, like disconnecting the hard drive cable. Even an incompetent shop would find that within several minutes of work. If they're trying to determine who's competent, do something relatively easy to fix, but hard to find, like damaging a cable. More data can be extracted from this based on what they charge you for, like if they replace a bunch of stuff before they find the real problem, do they charge you for those replacements?

You might also want to include a second problem that you don't report, like some seldom-used cable that gets unplugged, like the CD-ROM-to-audio-card cable in the PC World thing, though that one is less relevant these days. Maybe a floppy drive or memory card reader or front-panel USB cable.

That said, what are good difficult-to-diagnose, but easy-to-fix things? Bad hard drive cable. Underpowered power supply? Bad internal power cable? (Hm, that one could be dangerous.) Bad CPU fan? Bad CD-ROM drive. Heck, even a bad mouse or keyboard.

Of course, there are also software issues. Spyware. Virus. Corrupted drivers.

And BIOS things. Bad memory timing settings.

I feel like I should be able to come up with more stuff than this.