strace revealed the following:

fork()                                  = -1 EAGAIN (Resource temporar
+ily unavailable)
rt_sigprocmask(SIG_BLOCK, [CHLD], [RT_0], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [RT_0], NULL, 8) = 0
nanosleep({5, 0}, {5, 0})               = 0
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Which is really strange, what might cause the fork to fail for Apache requests?