Case in Order By Ignored without warning or error

Hello,

I'm currently working with PostGreSQL 9.3 on a RedHat 6.6 device.
One of my predecessors decided he wanted dynamic sorting which seems to be
ignored.

My made-up testing table definition is as follows:
CREATE TABLE films (
    id        SERIAL PRIMARY KEY,
    title       varchar(40) NOT NULL,
     imdbnumber     integer
);

INSERT INTO films (title, imdbnumber) VALUES ('Film a', 2000), ('Film b',
1999);

When using psql on the command line, I enter the following query:

select * from films order by (case 1 when 1 then 3 else 1 end);

I would expect this query to either sort on column 3, or refuse with an
error.
Instead it executes the query with incorrect sorting and no warning or
error.

Also when using indexes that are out of bounds, like -2 or 8, there are
neither warnings/errors nor sorting.

According to documentation,
https://www.postgresql.org/docs/9.3/static/queries-order.html, my select
query above is incorrect, however psql does not tell me this.

I don't see it as a problem, now that I know what PostGreSQL does, but I
would assume others without this knowledge would prefer at least a warning
or even an error.

Kind Regards,

Emiel Hermsen