When do we lose column names?

Consider the following query:

select (x).key as k, (x).value as v
from (select each(hstore(q)) as x     from (select oid, c.*           from pg_class c           where oid =
'parent'::regclass)q) t;
 

This gives results like this:
k  |   v
-----+--------f1  | 16410f2  | parentf3  | 2200f4  | 16412f5  | 0f6  | 10
....

Now add a limit clause to the innermost query:

select (x).key as k, (x).value as v
from (select each(hstore(q)) as x     from (select oid, c.*           from pg_class c           where oid =
'parent'::regclass          limit 99999999) q) t;
 

Now the result looks like this:
      k        |   v
----------------+--------oid            | 16410relam          | 0relacl         |relkind        | rrelname        |
parentreltype       | 16412
 


What I'm having difficulty understanding is why the limit clause should
make any difference.

Is this a bug? If not, is it documented.

cheers

andrew