Usually when I build source on a fast multiprocessor machine I use the -j switch to make the compiling process faster by increasing the number of jobs. Usually something like, make -j 4. Here’s an excerpt from the man page:

-j max_jobs
Specify the maximum number of jobs that make may have running at
any one time. Turns compatibility mode off, unless the -B flag
is also specified.

However, I noticed when doing my usual build from source for an upgrade my make -j 4 buildworld would fail with various compile errors. So I tried building without the -j switch and it worked. So while it takes a bit longer to compile the new world and new kernels, I’m not getting any more errors when upgrading FreeBSD 7.2 to FreeBSD 8.0.