apt-get install fails but then does not

gistfile1.txt

# apt-get -q -y install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1
Reading package lists...
Building dependency tree...
Reading state information...
The following extra packages will be installed:
  libcap2 libopts25
Suggested packages:
  ntp-doc
The following NEW packages will be installed:
  libcap2 libopts25 ntp
0 upgraded, 3 newly installed, 0 to remove and 62 not upgraded.
Need to get 684 kB of archives.
After this operation, 1,765 kB of additional disk space will be used.
WARNING: The following packages cannot be authenticated!
  libcap2 libopts25 ntp
E: There are problems and -y was used without --force-yes

# apt-get -q -y --force-yes install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1
# gets installed

# apt-get purge libcap2 libopts25 ntp

# was expecting this to fail
# apt-get -q -y install ntp=1:4.2.6.p3+dfsg-1ubuntu3.1
# but it goes through. is the --force-yes option remembered

asked question on http://askubuntu.com/questions/158549/apt-get-works-with-force-yes-but-cannot-reproduce-the-issue-on-a-fresh-box