When instrumentation is ON you can navigate from error stack trace to the code with a single mouse-click (very similar to ReSharper stack trace explorer).
When instrumentation is OFF then mouse-click functionality stops working because NCrunch-specific paths are used in this case. ReSharper Stack Trace Explorer still works fine and is able to handle stack traces from NCrunch correctly.
Original issue reports is here: http://forum.ncrunch.net/yaf_postst1513_Error-StackTrace-is-not-navigatable-when-instrumentation-is-off.aspx