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more than one row returned

Can anyone explain me why this query results in an error more than 1 row returned by a subquery used as an expression

select a.*, (select b.x from b where a.id = b.a_id) from a

select a.*, (select b.x from b where a.id = b.a_id) from a

especially since the problem is resolved once i change it to this

select a.*, case when (select true from b where a.id = b.a_id and b.x = true) = true then true else false end

select a.*, case when (select true from b where a.id = b.a_id and b.x = true) = true then true else false end