This is related to [93fb9f89d6a69dc3d8], though that prior ticket dealt
with joins instead of IN operators.  Consider:

<blockquote><verbatim>
CREATE TABLE b1(a TEXT); INSERT INTO b1 VALUES(1);
CREATE TABLE b2(x BLOB); INSERT INTO b2 VALUES(1);
SELECT count(*) FROM b1 WHERE a IN (SELECT x FROM b2);
SELECT count(*) FROM b1, b2 WHERE a=x;
CREATE UNIQUE INDEX b1a ON b1(a);
SELECT count(*) FROM b1 WHERE a IN (SELECT x FROM b2);
SELECT count(*) FROM b1, b2 WHERE a=x;


The code above used to return "0 0 1 1" prior to check-in [e72186f2d6]
which was the fix for ticket [93fb9f89d6a69dc3d8].  After that check-in
(circa 2009-08-13) the answer returned was "0 0 1 0".  The correct answer
should be "0 0 0 0".