Cause
The block size as stored in the control file header is different from the value of the initialization parameter DB_BLOCK_SIZE. This might be due to an incorrect setting of DB_BLOCK_SIZE, or else might indicate that the control file has either been corrupted or belongs to a different database.
Action
Restore a good copy of the control file. If the control file is known to be clean set the DB_BLOCK_SIZE to match control file headers block size value.