ERROR: could not open relation with OID 49152

OK, I'm getting the above error on one of my fairly new 8.3 production
databases. It happens when I run a query to see the size of my
tables.

SELECT pg_relation_size(c.relfilenode), n.nspname AS schemaname,
c.relname AS tablename, pg_get_userbyid(c.relowner) AS tableowner,
t.spcname AS tablespace, c.relhasindex AS hasindexes, c.relhasrules AS
hasrules, c.reltriggers > 0 AS hastriggers
FROM pg_class c
LEFT JOIN pg_namespace n ON n.oid = c.relnamespace
LEFT JOIN pg_tablespace t ON t.oid = c.reltablespace
WHERE n.nspname = 'public';
ERROR: could not open relation with OID 49152

If I run this query:

SELECT c.relfilenode, n.nspna... same as the above after this...

I get this:

relfilenode | schemaname | tablename | tableowner |
tablespace | hasindexes | hasrules | hastriggers
-------------+------------+----------------------------+------------+------------+------------+----------+-------------
49956 | public | paid_idx_pkey | dgish |
| f | f | f
49958 | public | page_access_timestamp | dgish |
| f | f | f
49152 | public | page_access | dgish |
| t | f | f

and a few other lines editted out for brevity.

Any ideas what the cause of this is? What part of my catalogs is borked?

Now, I can backup this db, and restore it with no errors, and the
error will go away. Same query on the same dataset returns the right
number of rows and their size.