Ocasionalmente (aproximadamente una vez al mes) una aplicación web ASP.NET MVC 3 falla con esta excepción.

System.NullReferenceException: Object reference not set to an instance of an object.

Server stack trace: 
   at System.Collections.Specialized.NameObjectCollectionBase.BaseGetAllKeys()
   at System.Collections.Specialized.NameValueCollection.get_AllKeys()
   at System.Web.WebPages.Scope.WebConfigScopeDictionary.<>c__DisplayClass4.<.ctor>b__0()
   at System.Lazy`1.CreateValue()

Exception rethrown at [0]: 
   at System.Lazy`1.get_Value()
   at System.Web.WebPages.Scope.WebConfigScopeDictionary.TryGetValue(Object key, Object& value)
   at System.Web.Mvc.ViewContext.ScopeGet[TValue](IDictionary`2 scope, String name, TValue defaultValue)
   at System.Web.Mvc.ViewContext.ScopeCache..ctor(IDictionary`2 scope)
   at System.Web.Mvc.ViewContext.ScopeCache.Get(IDictionary`2 scope, HttpContextBase httpContext)
   at System.Web.Mvc.ViewContext.GetClientValidationEnabled(IDictionary`2 scope, HttpContextBase httpContext)
   at System.Web.Mvc.Html.FormExtensions.FormHelper(HtmlHelper htmlHelper, String formAction, FormMethod method, IDictionary`2 htmlAttributes)
   at ASP._Page_Views_mywebpage_Create_cshtml.Execute() in c:\App\Views\Mywebpage\Create.cshtml:line 11
   at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
   at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
   at System.Web.WebPages.StartPage.ExecutePageHierarchy()
   at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
   at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
   at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19()
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult)
   at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
   at System.Web.Mvc.Controller.ExecuteCore()
   at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
   at System.Web.Mvc.MvcHandler.<>c__DisplayClass6.<>c__DisplayClassb.<BeginProcessRequest>b__5()
   at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0()
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Esta excepción ocurre en una línea con

@using (Html.BeginForm())

Desde el momento en que se produce esta excepción una vez, la aplicación no se puede utilizar. Cada vista comenzará a lanzar este error. He mirado alrededor en el código fuente pero parece que no puedo encontrar nada. Hay personas que experimentan el mismo comportamiento pero con unaerror diferente yo por código diferente.

Al profundizar en la fuente de WebConfigScopeDictionary, puede ver que se están leyendo los ajustes de la aplicación:

public WebConfigScopeDictionary() : this(WebConfigurationManager.AppSettings)
{
}
public WebConfigScopeDictionary(NameValueCollection appSettings)
{
    this._items = new Lazy<Dictionary<object, object>>(() => appSettings.AllKeys.ToDictionary((string key) => key, (string key) => appSettings[key], ScopeStorageComparer.Instance));
}

Las aplicaciones provienen del ConfigurationManager:

public static NameValueCollection AppSettings
{
    get
    {
        object section = ConfigurationManager.GetSection("appSettings");
        if (section == null || !(section is NameValueCollection))
        {
            throw new ConfigurationErrorsException(SR.GetString("Config_appsettings_declaration_invalid"));
        }
        return (NameValueCollection)section;
    }
}

Si hubiera un problema con web.config, este código debería haber lanzado una excepción. Pero no lo hace. Finalmente, se llama a count en la colección que devuelve la propiedad AppSettings. Dado que es probable que sea nulo en este punto se tira.

protected string[] BaseGetAllKeys()
{
    int count = this._entriesArray.Count;
    string[] array = new string[count];
    for (int i = 0; i < count; i++)
    {
        array[i] = this.BaseGetKey(i);
    }
    return array;
}

Si alguien tiene alguna idea, no dude en compartirla.