Hi I get an error at times with url re-write. here is my changes to config that fix this. Lines changed are in red

        Public Shared Function GetConfig() As RewriterConfiguration
            Dim config As RewriterConfiguration = DirectCast(DataCache.GetCache("RewriterConfig"), RewriterConfiguration)

            If (config Is Nothing) Then

                Dim filePath As String = HttpRuntime.AppDomainAppPath & "SiteUrls.config"

                ' Create a new Xml Serializer
                Dim ser As XmlSerializer = New XmlSerializer(GetType(RewriterConfiguration))

                Dim fileReader As FileStream = New FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read)

                ' Open up the file to deserialize
                Dim reader As StreamReader = New StreamReader(fileReader)

                ' Deserialize into RewriterConfiguration
                config = DirectCast(ser.Deserialize(reader), RewriterConfiguration)

                ' Close the Reader
                reader.Close()

                fileReader.Close()

                If File.Exists(filePath) Then
                    ' Create a dependancy on SiteUrls.config
                    Dim dep As CacheDependency = New CacheDependency(filePath)

                    ' Set back into Cache
                    DataCache.SetCache("RewriterConfig", config, dep)
                Else
                    Return config
                End If

            End If

            Return DirectCast(config, RewriterConfiguration)

        End Function

Here are two other thread describing the issues you see
Link 1
Link 2

The problem is that in get config, it will currently write the value to the data cache, which is fine. But it assumes that it will be still in the data cache at the end of the function, which is not always the case. So I store the result in to a var to make sure I can always return it. It just seems that multithread design was not taken in to account.