Briefly describe an error from your past projects that you have recently completed or an error from other projects which impress you most. State the reason, impact of the error and how did you find it.
遇到的问题：
- 正则表达式错误导致的匹配错误
- 经过几次不同的测试找到了bug，并修改了正确的表达式
- Ajax跨域请求
- 添加header
response['Access-Control-Allow-Origin'] = ''
response['Access-Control-Allow-Headers'] = ''