pg_dump fails with socket_not_open

Hi there folks,

I've just had pg_dump fail on me for the first time ever, and I'm not
sure why.

It generates 24MB of dump before bombing out with:

pg_dump: socket not open
pg_dump: SQL command to dump the contents of table "activity_log"
failed: PQendcopy() failed.
pg_dump: Error message from server: socket not open
pg_dump: The command was: COPY public.activity_log (<bunch of columns>
TO  stdout

Are there any common reasons for seeing this?

I'm fairly sure that the machine is ok hardware wise, and strangely
enough, pg_dumpall works, so it's really curiosity and to set my mind at
rest - any pointers appreciated.

Thanks,

Sarah.