Same Procedure runs and fails intermittently

HP Unix 11.0
Oracle 8.1.7.4
I have a procedure which runs OK sometimes and errs out at other times with an ORA-1843. This would have been expected to fail consistently if there was a date mismatch not only occasionally .The procedure was created with no errors.

The procedure in question is called via a shell script (if that helps any)

Your input will be greatly appreciated?

TY

if the code will be of any help the heart of the code is here

select (sysdate-to_date(t.start_time,'MM/DD/YY:HH24:MI:SS')) ,s.sid,s.serial#,
replace(substr(s.client_info,0,8),',')

   Name from v$transaction t,v$session s   where s.taddr=t.addr and to_date(t.start_time, 'DD/MM/YY:HH24:MI:SS') < (SYSDATE-(1/(96)))

              and substr(s.client_info,0,2) not in ('PS') Received on Wed May 26 2004 - 07:24:59 CDT