Not only on Rails, we sometimes encounter following error message of MySQL.
Illegal mix of collations (latin1_swedish_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '='.
So following commands may helps you.
NOTE: These commands' paths are based on Debian GNU/Linux.
Especially on Rails, `rake:reset` is the key point. Without this you cannot reset the tables.
Good luck.