Index: t/reopen_package.t
===================================================================
--- t/reopen_package.t	(revision 0)
+++ t/reopen_package.t	(revision 0)
@@ -0,0 +1,32 @@
+use Test::More tests => 9;
+use Test::Exception;
+
+# 1
+BEGIN { require_ok("MooseX::Adopt::Class::Accessor::Fast"); }
+
+use Class::MOP;
+use Class::Accessor::Fast;
+@My::Class::ISA = 'Class::Accessor::Fast';
+my $meta = Class::MOP::get_metaclass_by_name('My::Class') 
+  || Class::MOP::Class->initialize('My::Class');
+$meta->make_immutable;
+
+my @warnings;
+$SIG{__WARN__} = sub { push(@warnings, shift) };
+
+# 2-4
+lives_ok { My::Class->mk_accessors('foo') } 'mk_accessors on immutable';
+lives_ok { My::Class->mk_ro_accessors('quux') } 'mk_ro_accessors on immutable';
+lives_ok { My::Class->mk_wo_accessors('flibble') } 'mk_wo_accessors on immutable';
+
+# 5-7
+lives_ok { My::Class->make_accessor('bar') } 'mk_accessor on immutable';
+lives_ok { My::Class->make_ro_accessor('gong') } 'mk_ro_accessor on immutable';
+lives_ok { My::Class->make_wo_accessor('wibble') } 'mk_wo_accessor on immutable';
+
+# 8
+lives_ok { My::Class->follow_best_practice } 'follow_best_practice on immutable';
+
+# 9
+is( scalar(@warnings), 7, '7 warnings' );
+
Index: lib/MooseX/Emulate/Class/Accessor/Fast.pm
===================================================================
--- lib/MooseX/Emulate/Class/Accessor/Fast.pm	(revision 7132)
+++ lib/MooseX/Emulate/Class/Accessor/Fast.pm	(working copy)
@@ -3,6 +3,7 @@
 use Moose::Role;
 use Class::MOP ();
 use Scalar::Util ();
+use Carp ();
 
 our $VERSION = '0.00600';
 
@@ -75,6 +76,19 @@
     || Moose::Meta::Class->initialize($class);
 };
 
+my $reopen_package_if_needed = sub {
+  my $self = shift;
+  my $meta = $locate_metaclass->($self);
+  my $immutable = $meta->is_immutable;
+  if ($immutable) {
+    $meta->make_mutable;
+    my $class = Scalar::Util::blessed($self) || $self;
+    Carp::cluck("Class $class was immutable, but needs to be re-opened!");
+    return sub { $meta->make_immutable; };
+  }
+  return sub {};
+};
+
 sub BUILD {
   my $self = shift;
   my %args;
@@ -102,6 +116,7 @@
 sub mk_accessors{
   my $self = shift;
   my $meta = $locate_metaclass->($self);
+  my $reclose = $reopen_package_if_needed->($self);
   for my $attr_name (@_){
     my $reader = $self->accessor_name_for($attr_name);
     my $writer = $self->mutator_name_for( $attr_name);
@@ -122,6 +137,7 @@
       $meta->add_attribute($attr_name, @opts);
     }
   }
+  $reclose->();
 }
 
 =head2 mk_ro_accessors @field_names
@@ -133,6 +149,7 @@
 sub mk_ro_accessors{
   my $self = shift;
   my $meta = $locate_metaclass->($self);
+  my $reclose = $reopen_package_if_needed->($self);
   for my $attr_name (@_){
     my $reader = $self->accessor_name_for($attr_name);
     my @opts = ($meta->has_method($reader) ? () : (reader => $reader) );
@@ -142,6 +159,7 @@
         unless $meta->has_method("_${attr_name}_accessor");
     }
   }
+  $reclose->();
 }
 
 =head2 mk_ro_accessors @field_names
@@ -154,6 +172,7 @@
 sub mk_wo_accessors{
   my $self = shift;
   my $meta = $locate_metaclass->($self);
+  my $reclose = $reopen_package_if_needed->($self);
   for my $attr_name (@_){
     my $writer = $self->mutator_name_for($attr_name);
     my @opts = ($meta->has_method($writer) ? () : (writer => $writer) );
@@ -163,6 +182,7 @@
         unless $meta->has_method("_${attr_name}_accessor");
     }
   }
+  $reclose->();
 }
 
 =head2 follow_best_practices
@@ -175,11 +195,13 @@
 sub follow_best_practice{
   my $self = shift;
   my $meta = $locate_metaclass->($self);
+  my $reclose = $reopen_package_if_needed->($self);
 
   $meta->remove_method('mutator_name_for');
   $meta->remove_method('accessor_name_for');
   $meta->add_method('mutator_name_for',  sub{ return "set_".$_[1] });
   $meta->add_method('accessor_name_for', sub{ return "get_".$_[1] });
+  $reclose->();
 }
 
 =head2 mutator_name_for
@@ -236,30 +258,39 @@
 sub make_accessor {
   my($class, $field) = @_;
   my $meta = $locate_metaclass->($class);
+  my $reclose = $reopen_package_if_needed->($class);
   my $attr = $meta->find_attribute_by_name($field) || $meta->add_attribute($field); 
   my $reader = $attr->get_read_method_ref;
   my $writer = $attr->get_write_method_ref;
-  return sub {
+  my $accessor = sub {
     my $self = shift;
     return $reader->($self) unless @_;
     return $writer->($self,(@_ > 1 ? [@_] : @_));
-  }
+  };
+  $reclose->();
+  return $accessor;
 }
 
 
 sub make_ro_accessor {
   my($class, $field) = @_;
   my $meta = $locate_metaclass->($class);
+  my $reclose = $reopen_package_if_needed->($class);
   my $attr = $meta->find_attribute_by_name($field) || $meta->add_attribute($field); 
-  return $attr->get_read_method_ref;
+  my $method_ref = $attr->get_read_method_ref;
+  $reclose->();
+  return $method_ref;
 }
 
 
 sub make_wo_accessor {
   my($class, $field) = @_;
   my $meta = $locate_metaclass->($class);
+  my $reclose = $reopen_package_if_needed->($class);
   my $attr = $meta->find_attribute_by_name($field) || $meta->add_attribute($field); 
-  return $attr->get_write_method_ref;
+  my $method_ref = $attr->get_write_method_ref;
+  $reclose->();
+  return $method_ref;
 }
 
 1;