[Example 1]
create table grp (key int4);
insert into grp values(2);
insert into grp values(2);
create function grup() returns int4
as
'
declare
k	int4;
begin
select key into k from grp
group by key
having count(*) > 1;
return k;
end;
'
language 'plpgsql';
select grup();

The result must be 

grup
----
   2
(1 row)

but

grup
----
    
(1 row)

[Example 2]
create function subsel() returns int4
as
'
declare
k	int4;
begin
SELECT 1 AS one into k WHERE 1 IN (SELECT 1);
return k;
end;
'
language 'plpgsql';
select subsel();

The result must be

subsel
------
     1
(1 row)

but

pqReadData() -- backend closed the channel unexpectedly.
	This probably means the backend terminated abnormally before or while processing the request.
We have lost the connection to the backend, so further processing is impossible.  Terminating.