can't shrink relation

I am getting a NOTICE in logs about once a day:

NOTICE: Rel s: TID 212/16: DeleteTransactionInProgress 0 - can't
shrink relation

This is generated by vacuum. This does not happen after removing the
contents of the relation and re-inserting the data. I noticed this kind of
error in relation to a post on pg-general. Moreover, this error is
occuring in the same database I reported as saying 'Relation pg_shadow
does not exist' the other day.

oup=# select version();
version
---------------------------------------------------------------------
PostgreSQL 7.1.3 on i686-pc-linux-gnu, compiled by GCC egcs-2.91.66
(1 row)

Running on Linux 2.2 series kernel. The system has not crashed recently,
no disk failures, etc. In fact, postgres had been up since a few days
after 7.1.3 was released.

Thanks in advance,

Gavin