BUG #2393: update fails with unique constraint violation

The following bug has been logged online:

Bug reference: 2393
Logged by: Laurence Dawson
Email address: larry(dot)dawson(at)vanderbilt(dot)edu
PostgreSQL version: 8.1.3
Operating system: Ubuntu Dapper Drake
Description: update fails with unique constraint violation
Details:

Here is the table definition:

CREATE TABLE test.test
(
a int4 NOT NULL DEFAULT nextval('test.test_a_seq'::regclass),
CONSTRAINT pk PRIMARY KEY (a)
)
WITHOUT OIDS;
ALTER TABLE test.test OWNER TO lstore;

And then try an update:
lstore=> select * from test.test;
a
----
1
2
3
4
5
6
7
8
9
10
(10 rows)

lstore=> update test.test set a = a + 2 where a >= 3;
ERROR: duplicate key violates unique constraint "pk"
lstore=>