Why dos select work when pg_dump fails?

[Darren Reed <darrenr+postgres(at)fastmail(dot)net>]

For whatever reason, I'm back dealing with corrupted tables but
curiously while this fails:

pg_dump -U postgres -a -t table -f /data/table.dump

this works:

psql -U postgres -A -o /data/table.dump -c "SELECT * FROM table;"

Graned the format of the data is different, but regardless, the data is 
still there.

The message from pg_dump when it fails is:
pg_dump: SQL command failed
pg_dump: Error message from server: ERROR:  out of memory
DETAIL:  Failed on request of size 20.
pg_dump: The command was: SELECT t.tableoid, t.oid, t.relname as 
indexname, pg_catalog.pg_get_indexdef(i.indexrelid) as indexdef, 
t.relnatts as indnkeys, i.indkey, i.indisclustered, c.contype, 
c.conname, c.tableoid as contableoid, c.oid as conoid, (SELECT spcname 
FROM pg_catalog.pg_tablespace s WHERE s.oid = t.reltablespace) as 
tablespace FROM pg_catalog.pg_index i JOIN pg_catalog.pg_class t ON 
(t.oid = i.indexrelid) LEFT JOIN pg_catalog.pg_depend d ON (d.classid = 
t.tableoid AND d.objid = t.oid AND d.deptype = 'i') LEFT JOIN 
pg_catalog.pg_constraint c ON (d.refclassid = c.tableoid AND d.refobjid 
= c.oid) WHERE i.indrelid = '16456'::pg_catalog.oid ORDER BY indexname

Darren