Toast "unexpected chunk number" error

Summary:  User gets "unexpected chunk number" error on pg_dump
Serverity:  Serious
Version:  7.4.5
Platform: Not yet known
Description:
One of my clients has received an "unexpected chunk number" error on pg_dump
with a TOASTed column of a table.   This sounds suspiciously like an error
you dealt with on 7.3 last year (8/03) but the list archives are down and I
can't retrieve what Tom did to fix it/whether it was related to a PG bug.

ERROR:  unexpected chunk number 1 (expected 0) for toast value 126990852

We did some digging, and the problem does not appear to be the index on
pg_toast:

friend=# select chunk_seq,length(chunk_data) from pg_toast.pg_toast_17165
where chunk_id = 126990852; set enable_indexscan= false; select
chunk_seq,length(chunk_data) from pg_toast.pg_toast_17165 where chunk_id =
126990852;
 chunk_seq | length
-----------+--------
         1 |    608
(1 row)

SET
 chunk_seq | length
-----------+--------
         1 |    608


Thanks for advice.

--
Josh Berkus
Aglio Database Solutions
San Francisco