Update with subselect sometimes returns wrong result

Hi!

Given the following table:

CREATE TABLE t1 (id INTEGER);
INSERT INTO t1 VALUES (0), (1);

Then the following UPDATE should return exactly one row:

UPDATE t1 SET id = t1.id
FROM (SELECT id FROM t1 LIMIT 1 FOR UPDATE) AS subset
WHERE t1.id = subset.id
RETURNING t1.id

And it does so, most of of the time. But when run repeatedly in a loop like
in the attached script, then it will occasionally return 2 rows with two
different id values, something the LIMIT 1 should prevent. In my tests it
took from anywhere between 0 to 10 minutes and on average 1 to 2 minutes to
trigger the problem.

I have reproduced the issue on different machines and platforms with PG
9.3.1, 9.1.10, 9.0.14. (See file).

Interesting, and perhaps telling:
When autovacuum=off in postgresql.conf then I could not trigger the problem.

Oliver