> class virtual item =
>   object (self)
>     val mutable name = ""
>
>     method name = name
>
>     method set_name newname = name <- newname
>   end
> ;;
>
>
> class weapon =
>   object (self)
>     inherit item
>   end
> ;;
>
>
> class container =
>   object (self)
>     inherit item
>
>     val mutable items = []
>
>     method add newitem = items <- (newitem :: items)
>
>     method contents = items
>
>     method remove i = items <- List.filter (fun x -> x != i) items
>
>     method contents_to_string =
>       let print_item i =
>         match i with
>           | `Weapon w -> Printf.sprintf "%s (weapon)" w#name
>           | `Container c -> Printf.sprintf "%s (container) -
> Containing:\n%s" c#name c#contents_to_string
>       in
>         List.map print_item items
>
>   end
> ;;

First off, method contents_to_string has conflicting types to be
recursive. Second, adding a type constraint to items should fix your
problems.

My changes:

class container =
  .....
    val mutable items : [ `Weapon of weapon | `Container of container
] list = []
  ...
    method contents_to_string =
       .....
        List.fold_left (fun a b -> if a = "" then print_item b else a
^ "; " ^ print_item b) "" items

  end
;;

Jonathan