Magento出现这个问题最有可能是数据库因为非法重启关机等情况造成的。

去phpmyadmin里面把这几个表清空了，反正都是访问日志，一般这么处理后都会好。

TRUNCATE `sales_flat_quote`; 
ALTER TABLE `sales_flat_quote` AUTO_INCREMENT=1; 
TRUNCATE `sales_flat_quote_address`; 
ALTER TABLE `sales_flat_quote_address` AUTO_INCREMENT=1; 
TRUNCATE `sales_flat_quote_address_item`; 
ALTER TABLE `sales_flat_quote_address_item` AUTO_INCREMENT=1; 
TRUNCATE `sales_flat_quote_item`; 
ALTER TABLE `sales_flat_quote_item` AUTO_INCREMENT=1; 
TRUNCATE `sales_flat_quote_item_option`; 
ALTER TABLE `sales_flat_quote_item_option` AUTO_INCREMENT=1;
TRUNCATE `sales_flat_quote_payment`; 
ALTER TABLE `sales_flat_quote_shipping_rate` AUTO_INCREMENT=1;
 
TRUNCATE `log_customer`;
ALTER TABLE `log_customer` AUTO_INCREMENT=1;
TRUNCATE `log_quote`;
ALTER TABLE `log_quote` AUTO_INCREMENT=1;
TRUNCATE `log_summary`;
ALTER TABLE `log_summary` AUTO_INCREMENT=1;
TRUNCATE `log_summary_type`;
ALTER TABLE `log_summary_type` AUTO_INCREMENT=1;
TRUNCATE `log_url`;
ALTER TABLE `log_url` AUTO_INCREMENT=1;
TRUNCATE `log_url_info`;
ALTER TABLE `log_url_info` AUTO_INCREMENT=1;
TRUNCATE `log_visitor`;
ALTER TABLE `log_visitor` AUTO_INCREMENT=1;
TRUNCATE `log_visitor_info`;
ALTER TABLE `log_visitor_info` AUTO_INCREMENT=1;
TRUNCATE `report_event`;
ALTER TABLE `report_event` AUTO_INCREMENT=1;