Recently, I had to check out a linux tree at work from cvs. I only knew the repository, but didn't know the module to check out. So I made my best guess and started building the kernel. After plowing through a couple of little things, I hit the wall with:
scripts/Makefile.build:17: /pe/imp/xxx/linux-2.6.xxx/net/core/Makefile: No such file or directory
gmake: *** No rule to make target `/pe/imp/xxx/linux-2.6.xxx/net/core/Makefile'. Stop.
Guess what someone forgot to do when they were doing the cvs import :-)