While debugging an MVC4 app, I ran into the following error: System.MissingMethodException: Cannot create an abstract class. It was only happening for a single Action, and I wasn't able to set a breakpoint in that action. After some trial and error, I was able to find the issue in the code:

public virtual ActionResult Login(ActionResult redirectAction = null)
{
    ...
}

The goal was to have an T4MVC style helper for passing in an ActionResult instead of an untyped path. This was a bad idea, as ActionResult is an abstract class. When I hit http://localhost:2995/Account/Login, the MVC framework tried to instantiate ActionResult to fill in the default value for redirectAction. I simply reverted the parameter back to a string, and all was well.

public virtual ActionResult Login(string redirectPath = null)
{
    redirectPath = redirectPath ?? Url.Action(MVC.Home.Index());
    ...
}