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.