diff --git a/lib/check.ml b/lib/check.ml
index fee8fb19..6e093a0d 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -9041,7 +9041,7 @@ and unknown_name : 'a. ?setting:bool -> ctx -> Loc.t -> string -> 'a =
     let n = String.length name in
     let stem = if n > 2 then String.sub name 0 (n - 2) else "" in
     let suffix = if n > 2 then String.sub name (n - 2) 2 else "" in
-    if (suffix = "++" || suffix = "--") && lookup ctx stem <> None then
+    if (suffix = "++" || suffix = "--") && List.mem stem (value_candidates ctx) then
       Loc.failk "check/unknown-name" loc
         "unknown name %s — to %s %s, write %s"
         name (if suffix = "++" then "add one to" else "take one from") stem
diff --git a/lib/parse.ml b/lib/parse.ml
index 4651dd4c..400f894d 100644
--- a/lib/parse.ml
+++ b/lib/parse.ml
@@ -1972,7 +1972,22 @@ let rec decl (f : Form.t) : Ast.decl =
        rule against a suggestion that does not compile. The shapes alone
        then, which is what there is to say about a form that named too
        little. *)
+    let atom (x : Form.t) = match x.v with Form.List _ | Form.Vec _ | Form.Map _ -> false | _ -> true in
     (match args with
+     (* In a .fln file, the two definitions in its spelling. *)
+     | [ n; v ] when Source.indented_at f.loc && atom n ->
+       let rest =
+         Form.to_string n ^ " = " ^ (if atom v then Form.to_string v else "...") in
+       Loc.failk "parse/defvar-renamed" f.loc
+         "there is no defvar. Did you mean once? once %s initialises once and \
+          keeps its value; def %s re-initialises on every re-run" rest rest
+     | [ n; t; v ] when Source.indented_at f.loc && atom n && atom t ->
+       let rest =
+         Form.to_string n ^ ": " ^ Form.to_string t ^ " = "
+         ^ (if atom v then Form.to_string v else "...") in
+       Loc.failk "parse/defvar-renamed" f.loc
+         "there is no defvar. Did you mean once? once %s initialises once and \
+          keeps its value; def %s re-initialises on every re-run" rest rest
      | [] | [ _ ] ->
        Loc.failk "parse/defvar-renamed" f.loc
          "there is no defvar. Did you mean defonce? \
diff --git a/test/test_syntax.ml b/test/test_syntax.ml
index fb8c886d..0602f8f0 100644
--- a/test/test_syntax.ml
+++ b/test/test_syntax.ml
@@ -980,7 +980,11 @@ let () =
   refused "untyped-lambda.fln" "fn main() -> i32\n  let f = fn(a)\n    a\n  0\n"
     [ "let f: Fn(T, ...) -> R = fn(...)" ];
   refused "plusplus.fln" "fn main() -> i32\n  let x = 1\n  x++\n  x\n"
-    [ "write ++(x) or x += 1" ]
+    [ "write ++(x) or x += 1" ];
+  refused "plusplus-global.fln" "once g = 0\n\nfn main() -> i32\n  g--\n  0\n"
+    [ "write --(g) or g -= 1" ];
+  refused "defvar.fln" "defvar(x, 1)\n\nfn main() -> i32 = 0\n"
+    [ "once x = 1 initialises once"; "def x = 1 re-initialises" ]
 
 (* ── Both directions of an import, on both backends ────────────────── *)