SP2-1503: Unable to initialize Oracle call interface SP2-0152: ORACLE may not be functioning properly

For a new project I have both a 10g and a 11g database running on the same host.

Both the databases instances were up and running fine, the processes can be seen below

-bash-3.2$ ps -ef | grep test oracle   13727     1  0 Jan20 ?        00:00:00 ora_pmon_testteng oracle   13729     1  0 Jan20 ?        00:00:00 ora_psp0_testteng oracle   13731     1  0 Jan20 ?        00:00:01 ora_mman_testteng oracle   13733     1  0 Jan20 ?        00:00:00 ora_dbw0_testteng oracle   13735     1  0 Jan20 ?        00:00:00 ora_dbw1_testteng oracle   13737     1  0 Jan20 ?        00:00:00 ora_dbw2_testteng oracle   13739     1  0 Jan20 ?        00:00:00 ora_lgwr_testteng oracle   13741     1  0 Jan20 ?        00:00:13 ora_ckpt_testteng oracle   13743     1  0 Jan20 ?        00:00:05 ora_smon_testteng oracle   13745     1  0 Jan20 ?        00:00:00 ora_reco_testteng oracle   13747     1  0 Jan20 ?        00:00:03 ora_cjq0_testteng oracle   13749     1  0 Jan20 ?        00:00:01 ora_mmon_testteng oracle   13751     1  0 Jan20 ?        00:00:04 ora_mmnl_testteng oracle   13753     1  0 Jan20 ?        00:00:00 ora_d000_testteng oracle   13755     1  0 Jan20 ?        00:00:00 ora_s000_testteng oracle   13761     1  0 Jan20 ?        00:00:00 ora_qmnc_testteng oracle   13979     1  0 Jan20 ?        00:00:00 ora_q000_testteng oracle   14007     1  0 Jan20 ?        00:00:03 ora_q001_testteng oracle   29643     1  0 22:54 ?        00:00:00 ora_pmon_testdb1 oracle   29645     1  0 22:54 ?        00:00:00 ora_vktm_testdb1 oracle   29649     1  0 22:54 ?        00:00:00 ora_gen0_testdb1 oracle   29651     1  0 22:54 ?        00:00:00 ora_diag_testdb1 oracle   29653     1  0 22:54 ?        00:00:00 ora_dbrm_testdb1 oracle   29655     1  0 22:54 ?        00:00:00 ora_psp0_testdb1 oracle   29657     1  0 22:54 ?        00:00:00 ora_dia0_testdb1 oracle   29659     1  0 22:54 ?        00:00:01 ora_mman_testdb1 oracle   29661     1  0 22:54 ?        00:00:00 ora_dbw0_testdb1 oracle   29663     1  0 22:54 ?        00:00:00 ora_dbw1_testdb1 oracle   29665     1  0 22:54 ?        00:00:00 ora_dbw2_testdb1 oracle   29667     1  0 22:54 ?        00:00:00 ora_lgwr_testdb1 oracle   29669     1  0 22:54 ?        00:00:00 ora_ckpt_testdb1 oracle   29671     1  0 22:54 ?        00:00:00 ora_smon_testdb1 oracle   29673     1  0 22:54 ?        00:00:00 ora_reco_testdb1 oracle   29675     1  0 22:54 ?        00:00:00 ora_mmon_testdb1 oracle   29677     1  0 22:54 ?        00:00:00 ora_mmnl_testdb1 oracle   29679     1  0 22:54 ?        00:00:00 ora_d000_testdb1 oracle   29681     1  0 22:54 ?        00:00:00 ora_s000_testdb1 oracle   29704     1  0 22:54 ?        00:00:00 ora_qmnc_testdb1 oracle   29718     1  0 22:54 ?        00:00:00 ora_cjq0_testdb1 oracle   29729     1  0 22:54 ?        00:00:00 ora_q000_testdb1 oracle   29731     1  0 22:54 ?        00:00:00 ora_q001_testdb1 oracle   30227     1  0 23:04 ?        00:00:00 ora_smco_testdb1 oracle   30229     1  0 23:04 ?        00:00:00 ora_w000_testdb1 oracle   31137 27895  0 23:22 pts/3    00:00:00 grep test -bash-3.2$

Here I execute the .profile for the 10g instance where I set the ORACLE_HOME, ORACLE_SID and PATH. I then tried to connect using
sqlplus. As seen below everything worked fine, and I was able to connect.

-bash-3.2$ . ./.bash_profile_ten -bash-3.2$ echo $ORACLE_HOME /u01/app/oracle/product/10.2.0/dbhome_1 -bash-3.2$ echo $ORACLE_SID testteng -bash-3.2$ sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on Tue Jan 24 23:20:02 2012 Copyright (c) 1982, 2005, Oracle.  All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production With the Partitioning, OLAP and Data Mining options SQL>

Next I executed the .profile for the 11g instance where I set the ORACLE_HOME, ORACLE_SID and PATH. I then tried to connect using
sqlplus. As seen below I got the SP2-1503 and the SP2-0152 error messages

-bash-3.2$ . ./.bash_profile -bash-3.2$ echo $ORACLE_HOME /u01/app/oracle/product/11.2.0/dbhome_1 -bash-3.2$ echo $ORACLE_SID testdb1 -bash-3.2$ sqlplus / as sysdba SP2-1503: Unable to initialize Oracle call interface SP2-0152: ORACLE may not be functioning properly

Reason for the error message:
Find out what sqlplus you are using.

-bash-3.2$ which sqlplus /u01/app/oracle/product/10.2.0/dbhome_1/bin/sqlplus -bash-3.2$ -bash-3.2$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/u01/app/oracle/product/10.2.0/dbhome_1/bin:/u01/app/oracle/product/11.2.0/dbhome_1/bin

The above showed that the sqlplus I was using was from the 10g. This was because the PATH to the 10g bin was ahead of 11g bin in the PATH variable.

Fix:
Edit to .profile for the 11g and remove the 10g path from the PATH variable.
After the edit the PATH variable should look like below

-bash-3.2$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin::/u01/app/oracle/product/11.2.0/dbhome_1/bin -bash-3.2$ echo $ORACLE_HOME /u01/app/oracle/product/11.2.0/dbhome_1 -bash-3.2$ echo $ORACLE_SID testdb1 -bash-3.2$ sqlplus / as sysdba SQL*Plus: Release 11.2.0.1.0 Production on Tue Jan 24 23:57:17 2012 Copyright (c) 1982, 2009, Oracle.  All rights reserved. Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production With the Partitioning, OLAP, Data Mining and Real Application Testing options SQL>