postgresql tries to reuse plan but fails because the (dynamic) query has changed

Hi, all!

The following behavior found.

[ACTIONS]
execute the following code:
do $$
declare
     a text;
     cur record;
begin
     foreach a in array array['p'] loop
         for cur in execute 'select ' || a || ' r from
generate_series(3, 3) p' loop
             raise notice '%', cur.r;
         end loop;
     end loop;
     foreach a in array array['p||'',''||p*p'] loop
         for cur in execute 'select ' || a || ' r from
generate_series(3, 3) p' loop
             raise notice '%', cur.r;
         end loop;
     end loop;
     foreach a in array array['p', 'p||'',''||p*p'] loop
         for cur in execute 'select ' || a || ' r from
generate_series(3, 3) p' loop
             raise notice '%', cur.r;
         end loop;
     end loop;
end;
$$;

[EXPECTED]
NOTICE:  3
NOTICE:  3,9
NOTICE:  3
NOTICE:  3,9
DO

[RECEIVED]
NOTICE:  3
NOTICE:  3,9
NOTICE:  3
ERROR:  type of parameter 4 (text) does not match that when preparing
the plan (integer)
CONTEXT:  PL/pgSQL function inline_code_block line 18 at RAISE

[VERSION]
  PostgreSQL 9.3.1 on x86_64-unknown-linux-gnu, compiled by gcc (GCC)
4.4.7 20120313 (Red Hat 4.4.7-3), 64-bit
  Linux 2.6.32-358.el6.x86_64 #1 SMP Fri Feb 22 00:31:26 UTC 2013 x86_64
x86_64 x86_64 GNU/Linux

  also reproduced at postgresql 9.1.9, 9.3.2

[ALSO]
If current behavior is OK, please treat this bugreport as a bugreport on
documentation.
In what cases record variable is allowed or is not allowed to change its
structure?

Regards,
  Alexey Bashtanov