ORA-01102: cannot mount database in EXCLUSIVE mode

Error: ORA-01102: cannot mount database in EXCLUSIVE mode

SQL> startup
ORACLE instance started.

Total System Global Area 4993982464 bytes
Fixed Size                  2236928 bytes
Variable Size             922750464 bytes
Database Buffers         4060086272 bytes
Redo Buffers                8908800 bytes
ORA-01102: cannot mount database in EXCLUSIVE mode


Reason: database is started in EXCLUSIVE mode by default. Therefore, the 
ORA-01102 error is misleading and may have occurred due to one of the 
following reasons: 

- there is still an "sgadef.dbf" file in the "ORACLE_HOME/dbs" 
directory 
- the processes for Oracle (pmon, smon, lgwr and dbwr) still exist 
- shared memory segments and semaphores still exist even though the 
database has been shutdown 
- there is a "ORACLE_HOME/dbs/lk" file 

The "lk" and "sgadef.dbf" files are used for locking shared memory. 
It seems that even though no memory is allocated, Oracle thinks memory is 
still locked. By removing the "sgadef" and "lk" files you remove any knowledge
oracle has of shared memory that is in use. Now the database can start.

SQL>
SQL> shutdown immediate
ORA-01507: database not mounted


ORACLE instance shut down.
SQL> exit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@bachdb dbs]$ ll
total 28
-rw-rw---- 1 oracle oinstall 1544 Apr  2 02:28 hc_micrdb.dat
-rw-rw---- 1 oracle oinstall 1544 Apr 14 22:44 hc_MICRDB.dat
-rw-r----- 1 oracle oinstall  941 Apr 14 22:42 initMICRDB.ora
-rw-r--r-- 1 oracle oinstall 2851 May 15  2009 init.ora
-rw-r----- 1 oracle oinstall   24 Apr  2 02:26 lkMICRDB
-rw-r----- 1 oracle oinstall 1536 Apr  2 02:29 orapwmicrdb
-rw-r----- 1 oracle oinstall 2560 Apr 14 02:34 spfilemicrdb.ora

[oracle@bachdb dbs]$ rm -rf lkMICRDB
[oracle@bachdb dbs]$ sqlplus

SQL*Plus: Release 11.2.0.3.0 Production on Mon Apr 14 22:45:00 2014

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Enter user-name: /as sysdba
Connected to an idle instance.

SQL> startup
ORACLE instance started.

Total System Global Area 4993982464 bytes
Fixed Size                  2236928 bytes
Variable Size             922750464 bytes
Database Buffers         4060086272 bytes
Redo Buffers                8908800 bytes 
Database mounted.
Database opened.


SQL> exit