HEAD build failure

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hi,
HEAD autobuilds regulary fail with errors like this:
install ===> sys/modules/accf_httpready
x86_64--netbsd-install: 
/home/builds/ab/HEAD/amd64/200906070002Z-dest/stand/amd64: mkdir: File exists
x86_64--netbsd-install: 
/home/builds/ab/HEAD/amd64/200906070002Z-dest/stand/amd64/5.99.13/modules: 
mkdir: File exists
--- 
/home/builds/ab/HEAD/amd64/200906070002Z-dest/stand/amd64/5.99.13/modules/accf_dataready/accf_dataready.kmod
 ---
*** 
[/home/builds/ab/HEAD/amd64/200906070002Z-dest/stand/amd64/5.99.13/modules/accf_dataready/accf_dataready.kmod]
 Error code 1
1 error

nbmake: stopped in /home/builds/ab/HEAD/src/sys/modules/accf_dataready

(e.g. http://releng.netbsd.org/builds/HEAD/200906070002Z/amd64.build.failed).
It seems this doesn't always happens. Any idea what could cause this ?
Is it a parallel build issue, or something else ?

-- 
Manuel Bouyer, LIP6, Universite Paris VI.           
Manuel.Bouyer%lip6.fr@localhost
     NetBSD: 26 ans d'experience feront toujours la difference
--