invalid memory alloc request size

Yesterday I had a problem on a 64-bit 9.1.1 install:

# select version();
version
----------------------------------------------------------------------------------------------------------------
PostgreSQL 9.1.1 on x86_64-pc-linux-gnu, compiled by gcc-4.6.real (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1, 64-bit
(1 row)

The logs showed this anomaly:

2011-12-25T19:33:18+00:00 pgdb2-vpc postgres[27546]: [74474-1] ERROR: invalid memory alloc request size 18446744073709551613
2011-12-25T19:33:18+00:00 pgdb2-vpc postgres[27546]: [74474-2] STATEMENT: SELECT * FROM "asset_user_accesses" WHERE ("asset_user_accesses"."asset_code" = 'assignments:course_141208' AND "asset_user_accesses"."user_id" = 618503) LIMIT 1;

Googling around, it sounds like this is often due to table corruption, which would be unfortunate, but usually seems to be repeatable. I can re-run that query without issue, and in fact can select * from the entire table without issue. I do see the row was updated a few minutes after this error, so is it wishful thinking that vacuum came around and successfully removed the old, corrupted row version?