CASE Statement - Order of expression processing

I observed the following behaviour (I tested the following statements in
9.0.4, 9.0.5 and 9.3beta1):

$ psql template1
template1=# SELECT CASE WHEN 0=0 THEN 0 ELSE 1/0 END;
case
------
0
(1 row)

template1=# SELECT CASE WHEN 1=0 THEN 0 ELSE 1/0 END;
ERROR: division by zero

In this case the CASE behaves as expected.

But in the following expression:

template1=# SELECT CASE WHEN (SELECT 0)=0 THEN 0 ELSE 1/0 END;
ERROR: division by zero

(Just to be sure, a "SELECT (SELECT 0)=0;" returns true)

It seems that when the "CASE WHEN expression" is a query, the evaluation
order changes.
According to the documentation, this behaviour is wrong.

http://www.postgresql.org/docs/9.0/static/sql-expressions.html (4.2.13.
Expression Evaluation Rules):
"When it is essential to force evaluation order, a CASE construct (see
Section 9.16) can be used. "

http://www.postgresql.org/docs/9.0/static/functions-conditional.html(9.16.1.
CASE):
"If the condition's result is true, the value of the CASE expression is the
result that follows the condition, and the remainder of the CASE expression
is not processed."
"A CASE expression does not evaluate any subexpressions that are not needed
to determine the result."

Did I miss anything? Or is this really a bug?

Thanks,
Andrea Lombardoni