The shadow scriptlet has several problems.
First the output is a bit messed up, and when there are indeed group problems that are fixed, grpck returns 2, so pacman will report the scriptlet didn't execute properly :
http://bbs.archlinux.org/viewtopic.php?pid=482372#p482372

Besides that, grpck can display a warning but never fix it, for example :
'xavier' is a member of the 'power' group in /etc/group but not in /etc/gshadow
In this case, grpck returns 0. So we can decide whether to fix the file based on the return value.

So here are the changes I made :
1) output messed up -> quiet down everything
2) scriptlet failed to execute correctly -> added return 0
3) attempt to fix the file even when there is nothing grpck can fix -> use the return value of grpck to decide

post_upgrade() {
grpck -r &>/dev/null
if [ $? -eq 2 ]; then
echo "Fixing gshadow file ..."
while :; do echo "y"; done | grpck &>/dev/null
fi
return 0
}