I found an interesting problem this morning when my ASP.NET MVC application mysteriously broke after adding an HttpModule in the web.config. Here's the problem code:

public class MvcApplication : HttpApplication, IUnityContainerAccessor
{
    IUnityContainer container;
    public IUnityContainer Container
    {
        get { return container; }
    }
    public void Application_Start()
    {
        this.container = new UnityContainer();
        // this.container.RegisterTypes etc
    }

The container was being configured fine in Application_Start, but then UnityControllerFactory would throw "The container seems to be unavailable in your HttpApplication subclass" exceptions every time you tried to load a page -- this.container was somehow null again.

After doing a little digging and finding this article where someone had the same problem with Winsdor, it seems ASP.NET will create multiple HttpApplication instances when parallel requests are received. However, Application_Start only gets called once, so anything you would like to share between multiple instances (e.g. your IoC container) must be static:

public class MvcApplication : HttpApplication, IUnityContainerAccessor
{
    static IUnityContainer container; // good
    IUnityContainer container; // bad, not shared between HttpApplication instances
    public IUnityContainer Container
    {
        get { return container; }
    }