Strange duplicate key violation error

I have this table "replica_sync_log" which is updated thousands of
times a day to reflect the state of various schemas in a database
which acts as an offline secondary to various other databases (each
of the source databases is mapped to its own schema in the
secondary). The table has the following definition:

Table "radio_prod_default.replica_sync_log"
Column | Type | Modifiers
------------------+-----------------------------+-----------
db_host | text | not null
db_port | text | not null
db_name | text | not null
last_sync_id | integer |
last_sync_time | timestamp without time zone |
last_commit_time | timestamp without time zone |
Indexes:
"replica_sync_log_pkey" PRIMARY KEY, btree (db_host, db_port,
db_name), tablespace "data1"
Tablespace: "data1"

There are in fact one of these tables for each schema, each one
contains exactly one row (the "log" in the name is a bit misleading,
these just contain the current replica state, not a running log).

Here is an error that I got this morning that doesn't make much sense
to me:

2007-06-28 08:53:54.937 PDT [d:radio_reports_new u:slony s:4683d86f.
3681 3] ERROR: duplicate key violates unique constraint
"replica_sync_log_pkey"
2007-06-28 08:53:54.937 PDT [d:radio_reports_new u:slony s:4683d86f.
3681 4] STATEMENT: UPDATE replica_sync_log SET last_sync_id =
7147892, last_sync_time = '2007-06-27 23:28:04.586846'::timestamp,
last_commit_time = 'Thu Jun 28 08:53:54 2007'::timestamp WHERE
db_name = 'radio_prod_default' AND db_host = 'radiodb-default-1' AND
db_port = '5432';

I've never seen this error before in millions of updates to this
table. It confuses me because the columns indexed by the primary key
*are not* being modified, so how could the update violate the
constraint? Furthermore there is only one record in the table anyway.

The updates are performed by daemons that are in charge of
replicating the changes for one database each. So there is no
concurrent updating on this table, only one process updates it.

The error caused the daemon to pause, so I restarted it. The error
did not recur the second time.

fwiw this is postgres 8.1.8 on Linux.

Thanks for any insights.

-Casey