One thing that I know came up for me was exactly what the contract for some if the interfaces are. A specific example I can think of is RoutingHttpHandler. It tried to create my own implementation of this, but I remember having difficulties because the interface really isn't self explanatory, and there are no comments. From the bare interface it seems that

RoutingHttpHandler.match()

should always return

this

, but that isn't what the implementations do. When should it return

null

vs an HttpHandler that returns a 404?