In most cases, it is due to that reason, however, we need to dig into web test to find the cause.
The first thing to understand about a failing web test is that the request which fails is not always the cause of the problem. There is likely a previous request which is not behaving the same way it did during recording. It is also likely that
there are some dynamic variables, such as a session id, that are hard coded in your requests and you need to modify your requests to not hard code these values.
Here is an article offers detailed information about how to resolve this issue you can refer to: http://blogs.msdn.com/b/slumley/archive/2007/04/10/how-to-debug-a-web-test.aspx
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