diff --git a/src/Mutex.ml b/src/Mutex.ml
index 58866c2..689f365 100644
--- a/src/Mutex.ml
+++ b/src/Mutex.ml
@@ -4,6 +4,8 @@ sig
 
   val exclusively : (unit -> 'a) -> 'a
   val run : (unit -> 'a) -> 'a
+
+  val register_printer : ([`Exclusively] -> string option) -> unit
 end
 
 module Make () =
@@ -22,10 +24,16 @@ struct
       raise Locked
     else begin
       S.set true;
+      (* Favonia: I learn from the developers of eio that Fun.protect is not good at
+         preserving the backtraces. See https://github.com/ocaml-multicore/eio/pull/209. *)
       match f () with
       | ans -> S.set false; ans
       | exception e -> S.set false; raise e
     end
 
   let run f = S.run ~init:false f
+
+  let register_printer f = S.register_printer @@ fun _ -> f `Exclusively
+
+  let () = register_printer @@ fun _ -> Some "Unhandled algaeff effect; use Algaeff.Mutex.run"
 end
diff --git a/src/Mutex.mli b/src/Mutex.mli
index 7b011ad..d0673ae 100644
--- a/src/Mutex.mli
+++ b/src/Mutex.mli
@@ -49,6 +49,14 @@ sig
   (** [run f] executes the thunk [f] which may perform mutex effects.
       Each call of [run] creates a fresh mutex; in particular, calling [run] inside
       the thunk [f] will start a new scope that does not interfere with the outer scope. *)
+
+  val register_printer : ([`Exclusively] -> string option) -> unit
+  (** [register_printer p] registers a printer [p] via {!val:Printexc.register_printer} to convert unhandled internal effects into strings for the OCaml runtime system to display. Ideally, all internal effects should have been handled by {!val:run} and there is no need to use this function, but when it is not the case, this function can be helpful for debugging. The functor {!module:Modifier.Make} always registers a simple printer to suggest using {!val:run}, but you can register new ones to override it. The return type of the printer [p] should return [Some s] where [s] is the resulting string, or [None] if it chooses not to convert a particular effect. The registered printers are tried in reverse order until one of them returns [Some s] for some [s]; that is, the last registered printer is tried first. Note that this function is a wrapper of {!val:Printexc.register_printer} and all the registered printers (via this function or {!val:Printexc.register_printer}) are put into the same list.
+
+      The input type of the printer [p] is a variant representing internal effects used in this module. It corresponds to all the effects trigger by {!val:exclusively}.
+
+      @since 1.1.0
+  *)
 end
 
 module Make () : S
diff --git a/src/Reader.ml b/src/Reader.ml
index 7d713fc..1bf92bc 100644
--- a/src/Reader.ml
+++ b/src/Reader.ml
@@ -35,4 +35,6 @@ struct
   let register_printer f = Printexc.register_printer @@ function
     | Effect.Unhandled Read -> f `Read
     | _ -> None
+
+  let () = register_printer @@ fun _ -> Some "Unhandled algaeff effect; use Algaeff.Reader.run"
 end
diff --git a/src/Reader.mli b/src/Reader.mli
index 2e47ec4..72e5004 100644
--- a/src/Reader.mli
+++ b/src/Reader.mli
@@ -47,7 +47,7 @@ sig
 
       The input type of the printer [p] is a variant representation of the only internal effect used in this module. It corresponds to the effect trigger by {!val:read}.
 
-      @since 1.0.0
+      @since 1.1.0
   *)
 end
 
diff --git a/src/Sequencer.ml b/src/Sequencer.ml
index 3140f04..f5abd07 100644
--- a/src/Sequencer.ml
+++ b/src/Sequencer.ml
@@ -32,4 +32,6 @@ struct
   let register_printer f = Printexc.register_printer @@ function
     | Effect.Unhandled (Yield elt) -> f (`Yield elt)
     | _ -> None
+
+  let () = register_printer @@ fun _ -> Some "Unhandled algaeff effect; use Algaeff.Sequencer.run"
 end
diff --git a/src/Sequencer.mli b/src/Sequencer.mli
index b868961..1030e5c 100644
--- a/src/Sequencer.mli
+++ b/src/Sequencer.mli
@@ -41,7 +41,7 @@ sig
 
       The input type of the printer [p] is a variant representation of the internal effects used in this module. They correspond to the effects trigger by {!val:yield}. More precisely, [`Yield elt] corresponds to the effect triggered by [yield elt].
 
-      @since 1.0.0
+      @since 1.1.0
   *)
 end
 
diff --git a/src/State.ml b/src/State.ml
index 0b9eb50..54e8f3d 100644
--- a/src/State.ml
+++ b/src/State.ml
@@ -43,4 +43,6 @@ struct
     | Effect.Unhandled Get -> f `Get
     | Effect.Unhandled (Set state) -> f (`Set state)
     | _ -> None
+
+  let () = register_printer @@ fun _ -> Some "Unhandled algaeff effect; use Algaeff.State.run"
 end
diff --git a/src/State.mli b/src/State.mli
index fe38ef6..9c6ff23 100644
--- a/src/State.mli
+++ b/src/State.mli
@@ -48,7 +48,7 @@ sig
       - [`Get] corresponds to the effect triggered by [get ()].
       - [`Set state] corresponds to the effect triggered by [set state].
 
-      @since 1.0.0
+      @since 1.1.0
   *)
 end
 
diff --git a/src/UniqueID.ml b/src/UniqueID.ml
index 7194cfc..04fcc69 100644
--- a/src/UniqueID.ml
+++ b/src/UniqueID.ml
@@ -73,4 +73,6 @@ struct
     | Effect.Unhandled (Retrieve id) -> f (`Retrieve id)
     | Effect.Unhandled Export -> f `Export
     | _ -> None
+
+  let () = register_printer @@ fun _ -> Some "Unhandled algaeff effect; use Algaeff.UniqueID.run"
 end
diff --git a/src/UniqueID.mli b/src/UniqueID.mli
index 8da1cf3..a2bb788 100644
--- a/src/UniqueID.mli
+++ b/src/UniqueID.mli
@@ -63,7 +63,7 @@ sig
       - [`Retrieve id] corresponds to the effect triggered by [retrieve id].
       - [`Export] corresponds to the effect triggered by [export ()].
 
-      @since 1.0.0
+      @since 1.1.0
   *)
 end
 
diff --git a/src/Unmonad.ml b/src/Unmonad.ml
index 153db9b..5b61877 100644
--- a/src/Unmonad.ml
+++ b/src/Unmonad.ml
@@ -27,4 +27,8 @@ struct
       ; exnc = raise
       ; effc = function Monadic m -> Option.some @@ fun k -> M.bind m (Effect.Deep.continue k) | _ -> None
       }
+
+  let () = Printexc.register_printer @@ function
+    | Effect.Unhandled (Monadic _) -> Some "Unhandled algaeff effect; use Algaeff.Unmonad.run"
+    | _ -> None
 end
diff --git a/src/Unmonad.mli b/src/Unmonad.mli
index 2340ed6..2e7885b 100644
--- a/src/Unmonad.mli
+++ b/src/Unmonad.mli
@@ -1,8 +1,7 @@
 (** Effects for any monad (subject to OCaml continuations being one-shot). *)
 
 (** This is a general construction that uses effects to construct monadic expressions.
-    Here is an alternative implementation of {!module:State} using
-    the standard state monad:
+    Here is an alternative implementation of {!module:State} using the standard state monad:
 
     {[
       module StateMonad =
@@ -33,6 +32,8 @@
     raise an exception unless it encounters a truly unrecoverable fatal error. Raising an exception
     within [bind] will skip the continuation, and thus potentially skipping exception handlers
     within the continuation. Those handlers might be crucial for properly releasing acquired resources.
+
+    PS: We are not aware of any actual use of this module, but we decided to keep it anyway.
 *)
 
 module type Monad =