ORA-12518: TNS:listener could not hand off client connection
Cause: The process of handing off a client connection to another process failed.
Action: Turn on listener tracing and re-execute the operation. Verify that the listener and database instance are properly configured for direct handoff. If problem persists, call Oracle Support.

先试试 connect / as sysdba,看行不行，如果行的话，
再试试connect / @ as sysdba看看监听配置是否有问题。
如果这一步有问题，说明监听这块有问题。

1.cmd
2.sqlpls /nolog
3.connect sys/test as sysdba
没有问题
1.cmd
2.sqlplus /nolog
3.connect sys/test@test as sysdba
ORA-12518: TNS:listener could not hand off client connection
说明监听有问题

查到了相关资料如下：
http://www.dba-oracle.com/sf_ora_12518_tns_listener_could_not_hand_off_client_connection.htm

The reason ORA-12518 is being throw may be because of DEDICATED connections because Oracle 10g has a value of PROCESSES is defaulted at 150 which can be lower than necessary in a production system. Also, in pre-9i MTS, ORA-12518 may be thrown in SHARED SERVER because the dispatcher may have reached the maximum connection value, so it denies all other.

There are two solutions for ORA-12518 depending on which symptom you may be experiencing.
a.For the DEDICATED occurrence of ORA-12518, you would need to try increasing the PROCESSES parameter so that it can handle the needed number of processes. You can ensure that you have the needed value by monitoring the listener log for ORA-12518. Also, note that because the PROCESSES parameter is static, the database will need to be bounced.
b.If you are experiencing ORA-12518 because of a shared server issue, you first would need to use the command below to shutdown the dispatcher:
SQL> alter system shutdown immediate 'D001';
Then, add on new dispatchers:
SQL> alter system set DISPATCHERS = '(protocol=tcp)(dispatchers=3)';

通过以上资料分析得出可能出现问题的原因有两种：一是processes值设的过小，二是需要增加参数DISPATCHERS。

考虑到之前经常遇到ora-00020超出最大进程数的问题在安装数据库的时候已经把processes设到800了，为保险起见再show parameter processes

SQL> show parameter process;

NAME???????????????????????????????? TYPE??????? VALUE
------------------------------------ ----------- -----
aq_tm_processes????????????????????? integer???? 0
db_writer_processes????????????????? integer???? 1
gcs_server_processes???????????????? integer???? 0
job_queue_processes????????????????? integer???? 10
log_archive_max_processes??????????? integer???? 2
processes??????????????????????????? integer???? 1000

再查进程数

SQL> select count(*) from v$session;

? COUNT(*)
----------
?????? 224

所以最后得出结论应该不是processes的问题，应该用b方法来解决
1.connect sys/test as sysdba
2.show parameters dispatchers;
NAME??????????????? TYPE???????????? VALUE
dispatchers???????? string?????????? (protocol=tcp)(service=oracle10xdb)
max_dispatchers????? integer??
5.SQL>alter system set dispatchers = '(protocol=tcp)(dispatchers=3)(service=oracle10xdb)';
system altered
问题解决。
当然根据实际情况你也可以用a方法解决，

可以用show parameters processes;

命令查看processes的值，数据库在安装时默认为150，对于大型应用，32位的oracle一定要注意这个问题