ORA-12705 "invalid or unknown NLS parameter value specified"

The "Official" explanation is that the NLS parameter value specified in an alter session statement is not valid or not implemented. Except that this is not quite the entire story. If we put some nonsense in an alter session statement, Oracle is able to figure that out:


SQL> alter session set dodah = hohah;
alter session set dodah = hohah
*
ERROR at line 1:
ORA-02248: invalid option for ALTER SESSION

O.K., let's try a bad option to a good parameter:

SQL> alter session set nls_date_format = dodah;
ERROR:
ORA-01821: date format not recognized

What we need is a valid NLS parameter (such as nls_date_format, we were on the right track there) and pass it a bad option, but pick an NLS parameter which doesn't have it's own editing available (which appears to be any of them except the ones which refer to dates or time) :

SQL> alter session set nls_territory = dodah;
ERROR:
ORA-12705: invalid or unknown NLS parameter value specified

This describes how the ORA-12705 can arise in an existing session. Can one get this error during the logon process? The answer is, yes. Just set the NLS_LANG environment variable to an invalid value, and try to connect:

$ export NLS_LANG=dodah
$ echo $NLS_LANG
dodah
$ sqlplus psinger1@oradev

SQL*Plus: Release 9.2.0.8.0 - Production on Wed Oct 24 07:48:45 2007

Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.

Enter password:
ERROR:
ORA-12705: invalid or unknown NLS parameter value specified

and NLS_LANG appears to be the only NLS parameter which can be set as an environment variable. So, in summary, if you get a 12705 and you haven't just made an obvious mistake with an alter session statement, check your NLS_LANG setting.