diff -Naur ocaml-4.00.0/parsing/parser.mly ocaml-4.00.0.new/parsing/parser.mly
--- ocaml-4.00.0/parsing/parser.mly	2012-06-21 19:10:58.000000000 +0200
+++ ocaml-4.00.0.new/parsing/parser.mly	2012-10-01 19:30:59.521599707 +0200
@@ -177,6 +177,9 @@
   raise(Syntaxerr.Error(Syntaxerr.Unclosed(rhs_loc opening_num, opening_name,
                                            rhs_loc closing_num, closing_name)))
 
+let expecting pos nonterm =
+    raise Syntaxerr.(Error(Expecting(rhs_loc pos, nonterm)))
+
 let bigarray_function str name =
   mkloc (Ldot(Ldot(Lident "Bigarray", str), name)) Location.none
 
@@ -1251,6 +1254,8 @@
       { $1 }
   | pattern AS val_ident
       { mkpat(Ppat_alias($1, mkrhs $3 3)) }
+  | pattern AS error
+      { expecting 3 "identifier" }
   | pattern_comma_list  %prec below_COMMA
       { mkpat(Ppat_tuple(List.rev $1)) }
   | constr_longident pattern %prec prec_constr_appl
@@ -1259,10 +1264,16 @@
       { mkpat(Ppat_variant($1, Some $2)) }
   | pattern COLONCOLON pattern
       { mkpat_cons (ghpat(Ppat_tuple[$1;$3])) (symbol_rloc()) },
+  | pattern COLONCOLON error
+      { expecting 3 "pattern" }
   | LPAREN COLONCOLON RPAREN LPAREN pattern COMMA pattern RPAREN
       { mkpat_cons (ghpat(Ppat_tuple[$5;$7])) (symbol_rloc()) }
+  | LPAREN COLONCOLON RPAREN LPAREN pattern COMMA pattern error
+      { unclosed "(" 4 ")" 8 }
   | pattern BAR pattern
       { mkpat(Ppat_or($1, $3)) }
+  | pattern BAR error
+      { expecting 3 "pattern" }
   | LAZY simple_pattern
       { mkpat(Ppat_lazy $2) }
 ;
@@ -1303,6 +1314,8 @@
       { mkpat(Ppat_constraint($2, $4)) }
   | LPAREN pattern COLON core_type error
       { unclosed "(" 1 ")" 5 }
+  | LPAREN pattern COLON error
+      { expecting 4 "type" }
   | LPAREN MODULE UIDENT RPAREN
       { mkpat(Ppat_unpack (mkrhs $3 3)) }
   | LPAREN MODULE UIDENT COLON package_type RPAREN
@@ -1314,6 +1327,7 @@
 pattern_comma_list:
     pattern_comma_list COMMA pattern            { $3 :: $1 }
   | pattern COMMA pattern                       { [$3; $1] }
+  | pattern COMMA error                         { expecting 3 "pattern" }
 ;
 pattern_semi_list:
     pattern                                     { [$1] }
@@ -1668,6 +1682,8 @@
 val_ident:
     LIDENT                                      { $1 }
   | LPAREN operator RPAREN                      { $2 }
+  | LPAREN operator error                       { unclosed "(" 1 ")" 3 }
+  | LPAREN error                                { expecting 2 "operator" }
 ;
 operator:
     PREFIXOP                                    { $1 }
diff -Naur ocaml-4.00.0/parsing/syntaxerr.ml ocaml-4.00.0.new/parsing/syntaxerr.ml
--- ocaml-4.00.0/parsing/syntaxerr.ml	2012-03-23 03:16:44.000000000 +0100
+++ ocaml-4.00.0.new/parsing/syntaxerr.ml	2012-10-01 15:53:47.319445388 +0200
@@ -18,11 +18,13 @@
 
 type error =
     Unclosed of Location.t * string * Location.t * string
+  | Expecting of Location.t * string
   | Applicative_path of Location.t
   | Variable_in_scope of Location.t * string
   | Other of Location.t
 
 
+
 exception Error of error
 exception Escape_error
 
@@ -38,6 +40,10 @@
         fprintf ppf "%aThis '%s' might be unmatched"
           Location.print_error opening_loc opening
       end
+  | Expecting (loc, nonterm) ->
+      fprintf ppf
+        "%a@[Syntax error: %s expected.@]"
+        Location.print_error loc nonterm
   | Applicative_path loc ->
       fprintf ppf
         "%aSyntax error: applicative paths of the form F(X).t \
diff -Naur ocaml-4.00.0/parsing/syntaxerr.mli ocaml-4.00.0.new/parsing/syntaxerr.mli
--- ocaml-4.00.0/parsing/syntaxerr.mli	2012-03-23 03:16:44.000000000 +0100
+++ ocaml-4.00.0.new/parsing/syntaxerr.mli	2012-10-01 15:53:47.320445388 +0200
@@ -18,6 +18,7 @@
 
 type error =
     Unclosed of Location.t * string * Location.t * string
+  | Expecting of Location.t * string
   | Applicative_path of Location.t
   | Variable_in_scope of Location.t * string
   | Other of Location.t