In the Save As... dialog box in Mac OS X, it is possible to select an existing file, in which case that file will be overwritten. Just like Windows, in other words. (Is there a single feature Mac OS X has adopted from Windows that's good?) Actually it's more dangerous than the Windows equivalent, because there's no obvious indication that you've selected a file, and the filename field is at the top of the dialog, where you won't look after clicking on the file list below it. So it's easy to misclick and not notice that your carefully typed filename has just been replaced by the name of an existing file you do not want to lose. And if you thoughtlessly press return when asked "Are you sure you want to overwrite this file?", it will.
I had noticed this could be a problem, but I wasn't careful enough, and today I did it for the first time. Fortunately I was using Aquamacs, which helpfully made a backup of the file before clobbering it, so I didn't lose anything. (The backups can be annoying, but this is not the first time they've saved me. I should really be using a versioning filesystem.) But the same problem affects virtually every OS X application, and very few of them make backups. So here, in a system designed for ease of use, is a subtle feature with a rather unlikely intended use (usually the Save command does all the overwriting you want), and a very easy accidental use that loses data.
As the old joke goes:
How do you shoot yourself in the foot with Mac OS?
It's easy - just point and shoot!