Oracle error - ORA-10873 file 1 needs to be either taken out of backup mode or media recovered

If you got this error when trying to open an Oracle database

SQL> alter database open;
ORA-10873: file 1 needs to be either taken out of backup mode or media recovered
...


As the error message said, you need to move the file out of backup mode:

SQL> startup  mount ; //if the db is not mounted yet
SQL> alter database end backup ;

then you will be able to open the Oracle db:

SQL> alter database open ;