I know that seems like an odd suggestion and I *know* it is unlikely... I also realize my experience may be an anomaly, but allow me to regale you with this colorful anecdote:
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About 5 years ago, I was working with a PC. I would later discover it had a bad memory module, but at the time, the problem manifested itself in such a way that a bad hard drive seemed pretty likely. It wouldn't boot into Windows and always greeted you with a blue screen (ntfs.sys). It seemed pretty logical to run a drive diagnostic, which passed. Once I verified the drive was OK, I run chkdsk. Oddly, chkdsk started doing lots of 'hacking and slashing': 'correcting' index files, deleting attributes, orphaned files etc... Nothing out of the ordinary I guess, but it did this for several minutes which was definitely strange. When it was done, there was hardly anything left on the hard drive... sparse files sprinkled through the Windows folder, many programs were missing, user documents and desktop files were deleted and so on...
Fortunately, the data was of no consequence. I partitioned, formatted and reinstalled Windows XP. It installed without a hitch and performed a full, unconditional format just fine. After about a half hour or so of getting Windows setup, it blue screened while installing Office with an ntfs.sys error. It prompted me to run chkdsk again, so I did and it starting hacking and slashing files again... Once again, almost nothing was left.
Curious, I scanned the hard drive again with the manufacturer's diagnostic. It passed. I installed XP again. The same thing happened, but this time I didn't run chkdsk on the computer. I removed the hard drive and connected it to another XP system. A quick browse through the file system seemed just fine. I run chkdsk and it found errors, but they were fixed in a matter of seconds and there were not huge swaths of files that went 'missing'.
I put the drive back into the original computer, replaced the IDE cable, booted it up and after a period of time, I receved another ntfs.sys error. It prompted me for chkdsk and I did it -- It started 'fixing' tons and tons of files again. Yes, it deleted many, many files. At this point, I'm wondering what the significant correlation of chkdsk, ntfs.sys and massive data loss are.
I decided to do a memory test since I know that ntfs.sys errors can be a result of bad RAM. It failed the memory test. I isolated the memory module, removed it and partitioned, formatted and reinstalled XP again. The problems went away. I did a manual run of chkdsk on the system from recovery console and everything seemed to be working.
This blew my mind -- does this mean memory was responsible for the ntfs.sys? That seems likely, but chkdsk deleting data? Really? I had never seen anything like that before and at this point, I had serviced many, many, many, many computers. I was still skeptical though; I mean, maybe it was just a coincidence? I made an effort to find out by reinstalling the original memory, installed a different hard drive with a Windows XP install image on it. I booted the computer into recovery console using the XP CD and run chkdsk.... It started 'repairing' errors like crazy. I checked the aftermath and the file system was actually corrupted. I couldn't read it with the other computer.
I tried extra hard to get the computer to 'mess up' with the new RAM installed, but I couldn't. It worked just fine and it never came back me after it left the shop.
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Was this scientifically indisputable, empirical evidence? Not quite, but I like to think I did a pretty thorough job troubleshooting. Since then, I've certainly had data loss occur because of bad memory, but never because of bad memory + chkdsk. I still like to keep an open mind about it though, because it isn't worth losing someone's data and a relatively quick memtest isn't a bad idea anyway.
You also have valid points in making a backup first and the fact that chkdsk is more likely to screw things up than my situation above. I've grown somewhat complacent giving advice though, as I should have suggested backing up first and performing a drive diagnostic before doing a full on, 'destructive' chkdsk scan.
