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  | 16410
 f2  | parent
 f3  | 2200
 f4  | 16412
 f5  | 0
 f6  | 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            | 16410
 relam          | 0
 relacl         |
 relkind        | r
 relname        | parent
 reltype        | 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