In reference to the script below (I know it can be rewritten, that's not the point), I get 3 rows if the referenced index exists but only two rows if it does not. This is observable and repeatable just by dropping/creating the index. Drop the index and two rows are returned. Create the index, three rows are returned. Drop the index, two rows again. In addition, in no case does the selected column t2.c2 actually contain a value (it's always null). Since in the 3 row case, it returns a row with t1.c1=2, I would have expected a value from t2 (if you add t2.c1 to select clause you can see that is null as well).
It's probably worth mentioning (since it actually took me a while to notice) that the plans are subtlety different. Neither plan (with or without index existing) actually uses the index, but in one case there is an extra filter node.
version string is PostgreSQL 8.3.1 on i686-redhat-linux-gnu, compiled by GCC gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-33)
create table t1(c1 int);
create table t2(c1 int, c2 timestamp with time zone);
--problem index
create index someidx on t2 using btree(c2);
insert into t1 values (1),(2),(3);
insert into t2 values(2, now());
select
t1.c1,
t2.c2
from
t1
left join t2 on
t1.c1 = t2.c1
where
t2.c2 is null
or (
t2.c2 = (select max(c2) from t2 where t1.c1 = t2.c1)
and t2.c2 < now() - '1 day'::interval
);