Magento mysql backup may fail to restore with following error if mysql backup was taken using simple "mysqldump" command.
==========================================
ERROR 1005 (HY000) at line 4367: Can't create table 'db_name.core_session' (errno: 121)
ERROR 1005 (HY000) at line 4367: Can't create table 'db_name.core_store_group' (errno: 121)
==========================================
Solutions
1. Use a backup created using magento backup tool
2. Use the script http://files.bytehouse.co.uk/dump_magento_database.sh to create a mysql backup
===============================
wget http://files.bytehouse.co.uk/dump_magento_database.sh
bash dump_magento_database.sh
===============================
Just run script as "sh dump_magento_database.sh".
Script will ask for mysql user name, password, mysql hostname, database name
Once the details are provided, script will take backup
Credits
dump_magento_database.sh script is a modified version of https://gist.github.com/steve-ross/9958274
Thanks to https://gist.github.com/steve-ross