odd make/build output on ^Z / fg

Today I suspended a kernel build with ^Z to free up a bit of I/O bandwidth on 
a box.  When I re-foregrounded, all appeared fine for a bit, but later when I 
switched back to the xterm I found this:

ototypes  -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual  -Wundef 
-Wno-pointer-sign -fformat-extensions -nostdinc  -I. -I../../.. 
-I../../../contrib/altq -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include 
opt_global.h -fno-common -finline-limit=8000 --param inline-unit-growth=100 
--param large-function-growth=1000  -mno-align-long-strings 
-mpreferred-stack-boundary=2  -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 
-ffreestanding -fstack-protector -Werror  ../../../i386/acpica/acpi_wakeup.c
couldn't resume mac_pipe.o: No such process
*** Signal 1
couldn't resume audit_arg.o: No such process
*** Signal 1
couldn't resume nlm_prot_impl.o: No such process
*** Signal 1
couldn't resume nfs_serv.o: No such process
*** Signal 1
couldn't resume nfs_vnops.o: No such process
*** Signal 1
couldn't resume modules-obj: No such process
===> usb/uether (obj)
===> usb/aue (obj)
...
===> xfs (obj)
===> xl (obj)
===> zfs (obj)
===> zlib (obj)
*** Signal 1
6 errors

I've never seen that before, but I also don't suspend builds all that 
frequently.  New bug?  Old bug?

Robert N M Watson
Computer Laboratory
University of Cambridge