Oddity with NOT IN

I've got a customer that discovered something odd...

SELECT f1 FROM v1 WHERE f2 not in (SELECT bad FROM v2 WHERE f3 = 1);

does not error, even though bad doesn't exist, but

SELECT bad FROM v2 WHERE f3 = 1;
gives

ERROR: column "bad" does not exist

Is that expected?

This is on 9.4.8, and both v1 and v2 are views. The only "odd" thing
that I see is that v1 is a UNION ALL and v2 is a UNION. I don't think
there's any tables in common between the two views.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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