软件测试最大的困难？

今天在公司做了一个小范围的调查，主要是想了解一下大家在软件测试领域中所关注的地方。

调查的题目是：在你看来，软件测试最大的困难是什么？（如果大家有兴趣，可以回答一下。。）

经过最后的统计：

说需求不清晰的占了一般（但感觉这应该不是测试关注的地方，也可以说不是测试触及到的，但能反应出来需求的确是软件开发中最重要的环节）

其次是测试用例以及如何来衡量软件测试的质量

－－－5月9号补充一个国外的专家的回答－－－－

Q.What is the most challenging situation you had during testing?

I had posted the answer to this question on my blog as:
A. Vijay: When I switched my job some years back I was being asked
the same question.
Well a good answer to this question is depends on each ones
experience. If you came across any such a situation and found any
interesting Bug that was difficult to find out or analyzed any
project risk accurately before occurring then this could be the
answer to this question