Phobos's posix.mak has a rule:

$(DRUNTIME) :
        $(MAKE) -C $(DRUNTIME_PATH) -f posix.mak MODEL=$(MODEL)

which builds druntime if it's not already built.  However, this *also* builds .di files for druntime's src/import.

But none of the rules in phobos make any ddoc output depend on druntime.  So if I go to a clean phobos (with a clean dependent druntime), and do make -f posix.mak html, it fails with the following error:

../d-programming-language.org/std.ddoc -Df../d-programming-language.org/web/phobos/std_algorithm.html std/algorithm.d
std/c/string.d(12): Error: module string is in file 'core/stdc/string.d' which cannot be read

But if I make -f posix.mak release html, it works, because then druntime is first built.

Now, this is not necessarily a problem, because d-programming-language.org builds phobos using release html.  However, it builds with multiple jobs (-j 4).  It also cleans out druntime before building.  So what ends up happening is, druntime is a requirement for the release target, so running make for druntime consumes one job.  But it's not a requirement for the ddoc tasks, so it starts building those tasks.  If it gets to a ddoc file (such as std_algorithm.html) which requires druntime imports *before* druntime has finished building, the whole build fails.

This *might not* fail on other systems, depending on how fast druntime builds (it's a race condition).  But I think all ddoc files should depend on $(DRUNTIME).