Understanding "unknown" data type

I was working on a test restore and got:
WARNING:  column "astring" has type "unknown"

The restore worked correctly - it was a warning not an error - and we
located the cause and corrected it. But I did some playing leading to
questions for which I have been unable to locate answers:

steve=# create table foobar as select 'a string' as astring, null as anull;
WARNING:  column "astring" has type "unknown"
DETAIL:  Proceeding with relation creation anyway.
WARNING:  column "anull" has type "unknown"
DETAIL:  Proceeding with relation creation anyway.
SELECT 1

steve=# \d foobar
      Table "public.foobar"
  Column  |  Type   | Modifiers
---------+---------+-----------
  astring | unknown |
  anull   | unknown |

steve=# select * from foobar;
astring  | anull
----------+-------
  a string |

steve=# insert into foobar values ('1','2');
INSERT 0 1

steve=# select * from foobar;
  astring  | anull
----------+-------
  a string |
  1        | 2

steve=# select text (astring), text (anull) from foobar;
    text   | text
----------+------
  a string |
  1        | 2

So for starters...

1. Where can I read up on the purpose and properties of a data-type of
unknown? It apparently can be cast to some other types though "unknown"
is not shown in the output of \dC.

2. Is there any setting/option that would allow me to force an error
instead of allowing creation of tables with type unknown? (And why would
such a type be allowed in a table at all - it seems like a foot-gun
waiting to happen?)

Cheers,
Steve