In MySQL 5.1, online DDL operations such as adding/dropping index is
allowed. If the server crashes during online DDL operations, it seems
that the table definition of MySQL and that in the underlining storage
engine will be inconsistent. For example, if the server crashes in
handler::add_index when all the storage engine stuff has been done but
before returing to MySQL. So after restart, the storage engine will
think that the new index is ready, however, the MySQL server will
think that the new index is not ready.
So this seems to be a serious problem?