Description of problem:
There's a bug (reported as #541759) in yumex, which prevents abrt from being upgraded (or removed and installed again) in yumex.
You can avoid this by fixing the %pre section to look like this:
%pre
getent group abrt >/dev/null || groupadd -f --system abrt
exit 0

Moreover the %pre section will be according to
http://fedoraproject.org/wiki/Packaging/UsersAndGroups


Version-Release number of selected component (if applicable):
abrt-1.0.0-1.fc12.x86_64
yumex-2.9.4-1.fc12.noarch

How reproducible:
always (until #541759 gets fixed)

Steps to Reproduce:
1.run yumex
2.try to upgrade (or remove and install again) abrt
  
Actual results:
10:29:28 : YUM: /var/tmp/rpm-tmp.RNnPrf: line 1: /usr/sbin/groupadd: Permission denied

10:29:28 : YUM: Error in PREIN scriptlet in rpm package abrt-1.0.0-1.fc12.x86_64

10:29:28 : YUM: RPM Scriptlet: error: %pre(abrt-1.0.0-1.fc12.x86_64) scriptlet failed, exit status 126
10:29:28 : YUM: error:   install: %pre scriptlet failed (2), skipping abrt-1.0.0-1.fc12

Expected results:
abrt package is upgraded (installed)

Additional info:
This isn't concerning yum, only yumex.
See Bug #541759, comment #5