When the limit on the number of processes is reached, "make -j" still tries to launch additional processes.  This causes vfork to fail with EAGAIN, but the target whose commands failed in this way are neither redone nor marked with a clear error message.

Here's a transcript of a session from a Debian GNU/Linux box with Make 3.81beta4 (it might be necessary to play with the argument to "ulimit -u" to reproduce this, depending on the number of other processes that run under the user's UID):

    eliz@fencepost:~$ cat mkf4
    recurse: ; @$(MAKE) --no-print-directory -f mkf4 INC=yes all
    all: 0 1 2; @echo success

    0: ; @echo $(MAKEFLAGS)

    INC = no
    ifeq ($(INC),yes)
    -include 1.inc 2.inc
    endif

    1.inc: ; @echo ONE.inc; sleep 2; echo TWO.inc; echo '1: ; @echo ONE; sleep 2; echo TWO' > $@
    2.inc: ; @sleep 1; echo THREE.inc; echo '2: ; @sleep 1; echo THREE' > $@
    eliz@fencepost:~$ make -j -f mkf4
    --no-print-directory -j -- INC=yes
    ONE
    THREE
    TWO
    success
    eliz@fencepost:~$ ulimit -S -u 9
    eliz@fencepost:~$ make -j -f mkf4
    make[1]: vfork: Resource temporarily unavailable
    --no-print-directory -j -- INC=yes
    ONE
    TWO

As you see, after limiting the number of jobs, the commands for 2.inc are not run because vfork fails, but Make doesn't say  anything about failing to remake the target.