Looks like I don't have this fixed, so I have re-added it to the FAQ.
test=> create table xx (x int, y int) ;
CREATE
test=> insert into xx select usesysid, count(*) from pg_user group by
usesysid;
ERROR: The field being grouped by must appear in the target list
In this case, the group by the parser is checking for is x and y, not
the results of the select, so it fails.
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