The following example leads to an assertion from printtyp.ml when loaded in utop.  I made the example as small as I could, but unfortunately it still depends on a package called Future which in turn depends on Core.

To run it you'll have to do:

$ opam pin add future https://github.com/agarwal/future

Then hit Y to install the package.

$ cat tmp.ml 

open Future_std

let f items =

  Pipe.peek_deferred items >>= function

  | `Comment _ -> return ()

#require "future.std";;

# #use "tmp.ml";;

Fatal error: exception "Assert_failure typing/printtyp.ml:583:12"

Raised at file "bytes.ml", line 220, characters 25-34  

Doing the equivalent in the standard ocaml toploop or compiling with ocamlc/ocamlopt doesn't exhibit this assertion. You correctly get the real error reported:

$ ocamlfind ocamlc -package future.std -thread tmp.ml 

File "tmp.ml", line 5, characters 4-14:

Error: This pattern matches values of type [? `Comment of 'a ]

       but a pattern was expected which matches values of type

         ('b Future_std.Deferred.t as 'b) Future_std.Deferred.t

       The second variant type does not allow tag(s) `Comment

The assertion is also avoided if you replace `open Future_std` with a copy/paste of the relevant part of Future_std.