Receiving many more rows than expected

Hello,

I'm processing records in batches using PG 9.1.12, python 2.7, and psycopg
2.5.2 :

def enqueue_loop(q):
curs = DB_HANDLER.cursor()
query = """UPDATE foo SET processing = 't' WHERE id IN
(SELECT id FROM foo WHERE processing = 'f' ORDER BY id ASC LIMIT
5000 FOR UPDATE) RETURNING *"""
while DO_LOOP: #the whole program eventually stops if this is false
results = curs.fetchall()
rlen = len(results)
if rlen > 0:
LOG.debug("Fetched %d rows", rlen)
if rlen == 0:
# [...] wait for notification...
continue
# [...] Enqueue batch and let other threads process it.
# [...] Those threads will eventually delete the processed rows from
# the foo table.

The problem is that sometimes (once every few days at about 2-300K queries per
day) I get many more rows than the max 5000 I asked for (I've seen up to 25k).
And I'm getting timeouts and other problems as a result.

The id column is your typical primary key integer with a unique index. I've
checked the problematic cases and there are no id gaps or duplicate rows.
There are multiple threads in the program, but only the main thread is running
enqueue_loop(). I'm not sure if this is a server or a driver issue.

Any idea ? Thanks.

--
Vincent de Phily