For most, after 10.5.3 has been installed, the file “mach_kernel.ctfsys” is invisible. A quick check of the Finder does not (normally) identify such a critter. And that's how it should be.
But if you drag the icon of your hard drive to the Dock, and then open that, the mach_kernel.ctfsys file is (now) visible.
Additionally, when others are connected to your machine via your network, they too will be able to see the mach_kernel.ctfsys file. Thus, they are able, either innocently or maliciously, to cause harm.
1. At the prompt, enter (but without the quotes
“sudo chflags hidden /mach_kernel.ctfsys” (you may want to cut & paste this)
2. You will then need to enter your password. Note: even though you enter your PW, there is no indication of such, but don’t worry about it.
3. After your PW has been entered, hit Return.
4. Quit Terminal.
That’s it. AFAIK, this fix has no side affects. Indeed it worked for me. Still, it is best to have a verified backup.
Others, with far more smarts, please add detail, explanations of why/how this happens, potential "gotchas", etc.