what causes kernel header files to disappear?

[Pollywog <pollywog@shadypond.com>]

Every so often, when I try to compile something, I find that some header file
in /usr/src/linux/include/linux  OR
/usr/src/linux/include/net  is missing and I need to rm -rf the kernel sources
and replace them with a fresh tarball.

What causes the headers to just disappear when I did not mess with them?
Does this happen to anyone else?  It happened to me a few weeks ago and again
today:

Making dependencies: please wait...
Something wrong... deleting dependencies.
make: *** [include/net/if.h] Error 1


thanks

--
Andrew