Same thing happened with PS 5.5.32

root@debian:~# sudo -i DEBIAN_FRONTEND=noninteractive apt-get install percona-server-server-5.5 percona-server-client-5.5
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libhtml-template-perl
Use 'apt-get autoremove' to remove them.
The following extra packages will be installed:
  percona-server-common-5.5
Suggested packages:
  tinyca
The following packages will be REMOVED:
  percona-server-client-5.6 percona-server-common-5.6 percona-server-server-5.6
The following NEW packages will be installed:
  percona-server-client-5.5 percona-server-common-5.5 percona-server-server-5.5
0 upgraded, 3 newly installed, 3 to remove and 0 not upgraded.
1 not fully installed or removed.
Need to get 30.1 MB of archives.
After this operation, 21.3 MB disk space will be freed.
Do you want to continue [Y/n]? Y
WARNING: The following packages cannot be authenticated!
  percona-server-common-5.5 percona-server-client-5.5 percona-server-server-5.5
Install these packages without verification [y/N]? y
Get:1 http://repo.percona.com/apt/ squeeze/main percona-server-common-5.5 amd64 5.5.32-rel31.0-549.squeeze [2,760 B]
Get:2 http://repo.percona.com/apt/ squeeze/main percona-server-client-5.5 amd64 5.5.32-rel31.0-549.squeeze [8,543 kB]
Get:3 http://repo.percona.com/apt/ squeeze/main percona-server-server-5.5 amd64 5.5.32-rel31.0-549.squeeze [21.6 MB]                          
Fetched 30.1 MB in 1min 31s (331 kB/s)                                                                                                        
Preconfiguring packages ...
(Reading database ... 130321 files and directories currently installed.)
Removing percona-server-server-5.6 ...
Stopping MySQL (Percona Server): mysqld/usr/sbin/mysqld: Can't read dir of '/etc/mysql/conf.d/' (Errcode: 2 - No such file or directory)
Fatal error in defaults handling. Program aborted
/usr/sbin/mysqld: Can't read dir of '/etc/mysql/conf.d/' (Errcode: 2 - No such file or directory)
Fatal error in defaults handling. Program aborted
.
Removing percona-server-client-5.6 ...
Removing percona-server-common-5.6 ...
Processing triggers for man-db ...
Processing triggers for menu ...
Selecting previously deselected package percona-server-common-5.5.
(Reading database ... 130067 files and directories currently installed.)
Unpacking percona-server-common-5.5 (from .../percona-server-common-5.5_5.5.32-rel31.0-549.squeeze_amd64.deb) ...
Selecting previously deselected package percona-server-client-5.5.
Unpacking percona-server-client-5.5 (from .../percona-server-client-5.5_5.5.32-rel31.0-549.squeeze_amd64.deb) ...
Selecting previously deselected package percona-server-server-5.5.
Unpacking percona-server-server-5.5 (from .../percona-server-server-5.5_5.5.32-rel31.0-549.squeeze_amd64.deb) ...
Aborting downgrade from (at least) 5.6 to 5.5.
If are sure you want to downgrade to 5.5, remove the file
/var/lib/mysql/debian-*.flag and try installing again.
dpkg: error processing /var/cache/apt/archives/percona-server-server-5.5_5.5.32-rel31.0-549.squeeze_amd64.deb (--unpack):
 subprocess new pre-installation script returned error exit status 1
configured to not write apport reports
                                      Processing triggers for man-db ...
Processing triggers for menu ...
Errors were encountered while processing:
 /var/cache/apt/archives/percona-server-server-5.5_5.5.32-rel31.0-549.squeeze_amd64.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@debian:~#