ERROR:
ORA-12705: Cannot access NLS data files or invalid environment specified

When logging into oracle using sqlplus:

sqlplus '/ as sysdba';

Message 133 not found;  product=SQLPlus; facility=SP2
: Release 9.2.0.6.0 - Production on Tue Apr 7 14:20:15 2009

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ERROR:
ORA-12705: Cannot access NLS data files or invalid environment specified

oerr ora 12705 shows the following:

12705, 00000, "Cannot access NLS data files or invalid environment specified"
// *Cause: Either an attempt was made to issue an ALTER SESSION command
//         with an invalid NLS parameter or value; or the environment
//         variable(s) NLS_LANG, ORA_NLSxx, or ORACLE_HOME was incorrectly
//         specified, therefore the NLS data files cannot be located.
// *Action: Check the syntax of the ALTER SESSION command and the NLS
//          parameter, correct the syntax and retry the statement, or
//          specify the correct directory path/values in the environment
//          variables.

In my case this was very easy.  My ORACLE_HOME was set to /u01/oracle/product/9.2.0.6 instead of /u01/oracle/product/10.2.0.3.    Once I corrected that it went through just fine.   But that may not be the only cause of this error. 

Check that any of the following is true:

NLS_LANG parameter is correct or any other ORA_NLSxx is correct.

ORACLE_HOME parameter is set correct.

Check to see that you are in the correct $ORACLE_HOME during migration etc..

You can use the unset command in unix to unset your parameter or simply set/export them again.