Why my Postgres 9.3 database is logging crazily autovacuum alerts
It's a temporary table that wasn't removed automatically when it should have.

The fix from the point of view of a DBA is to simply drop the table manually.

There are several references to this problem on mailing lists. See for instance
[bug fix] Suppress "autovacuum: found orphan temp table" message
for a discussion on this subject.