This seems a known issue...
But hitting with this exception frequently.
Suddenly a pop window came up with the following exception:

See the end of this message for details on invoking
just-in-time (JIT) debugging instead of this dialog box.

• • • • • • • • • • • • • • Exception Text **************
                            System.InvalidCastException: Unable to cast COM object of type 'System.__ComObject' to interface type 'mshtml.IHTMLElement'. This operation failed because

the QueryInterface call on the COM component for the interface with IID '{3050F1FF-98B5-11CF-BB82-00AA00BDCE0B}' failed due to the following error:

The RPC server is unavailable. (Exception from HRESULT: 0x800706BA).
at Script_assistant.MsHtmlView.MapElements(HTMLDocument mDoc, String parUid) in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 187
at Script_assistant.MsHtmlView.SyncBrowserView(Int32 windowHandle) in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 172
at Script_assistant.MsHtmlView.SyncWithBrowser(Int32 windowHandle) in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 66
at Script_assistant.MsHtmlView.SyncWithBrowser() in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 61
at Script_assistant.MsHtmlView.RefreshBrowser() in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 338
at Script_assistant.MsHtmlView.timer1_Tick(Object sender, EventArgs e) in C:\svn_wet\main\openqa\src\MsHtmlView.cs:line 528
at System.Windows.Forms.Timer.OnTick(EventArgs e)
at System.Windows.Forms.Timer.TimerNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

• • • • • • • • • • • • • • Loaded Assemblies **************
                            mscorlib
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
                            ----------------------------------------
                            wet_script_assistant
                            Assembly Version: 0.9.8.0
                            Win32 Version: 0.9.8.0
                            CodeBase: file:///C:/Qantom/WET/bin/wet_script_assistant.exe
                            ----------------------------------------
                            log4net
                            Assembly Version: 1.2.10.0
                            Win32 Version: 1.2.10.0
                            CodeBase: file:///C:/Qantom/WET/bin/log4net.DLL
                            ----------------------------------------
                            System.Configuration
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_MSIL/System.Configuration/2.0.0.0__b03f5f7f11d50a3a/System.Configuration.dll
                            ----------------------------------------
                            System
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
                            ----------------------------------------
                            System.Xml
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_MSIL/System.Xml/2.0.0.0__b77a5c561934e089/System.Xml.dll
                            ----------------------------------------
                            System.Windows.Forms
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
                            ----------------------------------------
                            System.Drawing
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
                            ----------------------------------------
                            CookComputing.XmlRpcV2
                            Assembly Version: 0.0.0.0
                            Win32 Version: 0.0.0.0
                            CodeBase: file:///C:/Qantom/WET/bin/CookComputing.XmlRpcV2.DLL
                            ----------------------------------------
                            XmlRpcProxy1b69b713-78d7-4100-8ebc-29d33f315c43
                            Assembly Version: 0.9.8.0
                            Win32 Version: 0.0.0.0
                            CodeBase: file:///C:/Qantom/WET/bin/CookComputing.XmlRpcV2.dll
                            ----------------------------------------
                            Interop.SHDocVw
                            Assembly Version: 1.1.0.0
                            Win32 Version: 1.1.0.0
                            CodeBase: file:///C:/Qantom/WET/bin/Interop.SHDocVw.DLL
                            ----------------------------------------
                            Microsoft.mshtml
                            Assembly Version: 7.0.3300.0
                            Win32 Version: 7.0.3300.0
                            CodeBase: file:///C:/Qantom/WET/bin/Microsoft.mshtml.DLL
                            ----------------------------------------
                            CustomMarshalers
                            Assembly Version: 2.0.0.0
                            Win32 Version: 2.0.50727.42 (RTM.050727-4200)
                            CodeBase: file:///C:/WINNT/assembly/GAC_32/CustomMarshalers/2.0.0.0__b03f5f7f11d50a3a/CustomMarshalers.dll
                            ----------------------------------------
                            HtmlEditor
                            Assembly Version: 1.0.2424.22428
                            Win32 Version: 1.0.2424.22428
                            CodeBase: file:///C:/Qantom/WET/bin/HtmlEditor.DLL
                            ----------------------------------------

• • • • • • • • • • • • • • JIT Debugging **************
                            To enable just-in-time (JIT) debugging, the .config file for this
                            application or computer (machine.config) must have the
                            jitDebugging value set in the system.windows.forms section.
                            The application must also be compiled with debugging
                            enabled.

For example:

<configuration>
<system.windows.forms jitDebugging="true" />
</configuration>

When JIT debugging is enabled, any unhandled exception
will be sent to the JIT debugger registered on the computer
rather than be handled by this dialog box.