mysql> show variables like 'tmp_table_size';
+---------------------------------+----------------------+
| Variable_name | Value |
+---------------------------------+----------------------+
| tmp_table_size | 16777216 |
+---------------------------------+----------------------+

我改了my.cnf文件

mysql> show variables like 'max_heap_table_size';
+---------------------------------+----------------------+
| Variable_name | Value |
+---------------------------------+----------------------+
| max_heap_table_size | 671088 |
+---------------------------------+----------------------+

mysql> show variables like 'tmp_table_size';
+---------------------------------+----------------------+
| Variable_name | Value |
+---------------------------------+----------------------+
| tmp_table_size | 671088 |
+---------------------------------+----------------------+

顺便看一下这篇文章中的一段话：
http://dev.mysql.com/tech-resources/articles/mysql-query-cache.html

Qcache_hits and Qcache_inserts shows the number of times a query was serviced from the cache and how many queries have been inserted into the cache. Low ratios of hits to inserts indicate little query reuse or a too-low setting of the query_cache_limit, which serves to govern the RAM devoted to each inpidual query cache entry. Large query result sets will require larger settings of this variable.

补充：
今天在手册上看到这段话

http://dev.mysql.com/doc/refman/5.1/en/out-of-memory.html
If you issue a query using the mysql client program and receive an error like the following one, it means that mysql does not have enough memory to store the entire query result:

所以我把我的所有存储过程都修改了。
因为里面的预处理语句没有清零。
经过测试。
比如：
里面用到
PREPARE S1 FROM @STMT;
...
一定要
SET @STMT = '';
不知道这个是不是根本原因。我会再次更新的。
再次证明这样做会减少这种情况。不过下午把所有存储过程修改了。
凡是普通连接的都改为LEFT JOIN 了。通过EXPLAIN建立了相关索引。
而且把版本换成了5.0.45。至于是否还是出现错误。还在继续关注中。。目前没有任何错误。

这个问题貌似MYSQL官方解决了。
http://bugs.mysql.com/bug.php?id=318bitsCN.com