Comment 3 for bug 1206648

And same with PS 5.1.70

root@debian:~# sudo -i DEBIAN_FRONTEND=noninteractive apt-get install percona-server-server-5.1 percona-server-client-5.1
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  percona-server-common
Suggested packages:
  tinyca
The following packages will be REMOVED:
  percona-server-client-5.5 percona-server-common-5.5
The following NEW packages will be installed:
  percona-server-client-5.1 percona-server-common percona-server-server-5.1
0 upgraded, 3 newly installed, 2 to remove and 0 not upgraded.
2 not fully installed or removed.
Need to get 0 B/24.6 MB of archives.
After this operation, 131 MB of additional disk space will be used.
Do you want to continue [Y/n]? Y
WARNING: The following packages cannot be authenticated!
  percona-server-common percona-server-client-5.1 percona-server-server-5.1
Install these packages without verification [y/N]? y
Preconfiguring packages ...
(Reading database ... 130109 files and directories currently installed.)
Removing percona-server-client-5.5 ...
Removing percona-server-common-5.5 ...
Processing triggers for menu ...
Processing triggers for man-db ...
Selecting previously deselected package percona-server-common.
(Reading database ... 130066 files and directories currently installed.)
Unpacking percona-server-common (from .../percona-server-common_5.1.70-rel14.8-580.squeeze_amd64.deb) ...
Selecting previously deselected package percona-server-client-5.1.
Unpacking percona-server-client-5.1 (from .../percona-server-client-5.1_5.1.70-rel14.8-580.squeeze_amd64.deb) ...
Processing triggers for menu ...
Processing triggers for man-db ...
Setting up percona-server-common (5.1.70-rel14.8-580.squeeze) ...
Selecting previously deselected package percona-server-server-5.1.
(Reading database ... 130116 files and directories currently installed.)
Unpacking percona-server-server-5.1 (from .../percona-server-server-5.1_5.1.70-rel14.8-580.squeeze_amd64.deb) ...
Aborting downgrade from (at least) 5.6 to 5.1.
If are sure you want to downgrade to 5.1, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing /var/cache/apt/archives/percona-server-server-5.1_5.1.70-rel14.8-580.squeeze_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
configured to not write apport reports
                                      Errors were encountered while processing:
 /var/cache/apt/archives/percona-server-server-5.1_5.1.70-rel14.8-580.squeeze_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@debian:~#