Commented by: Maxim Kuzmin (cybermax)

For example, this query stopping FB 2.5.6 with bugcheck:
SELECT
SUM(SUM((SELECT SUM(RF.RDB$FIELD_POSITION) FROM RDB$RELATION_FIELDS RF)))
FROM
RDB$DATABASE

Commented by: @pavel-zotov

Check following sample:

set list on;

-- query from ticket:
select sum(sum((select sum(rf.rdb$field_position) from rdb$relation_fields rf))) as sf from rdb$database;
--select sign( sum( sum( ( select sum(rf.rdb$field_position) from rdb$relation_fields rf) ) ) ) as sf from rdb$database;

set echo on;

select sum( 1 ) as s01 from rdb$database rd;
select sum( rf.rdb$field_position ) as s01 from rdb$relation_fields rf;

----------------------------------------------------------------------

select sum( ( select sum( 1 ) from rdb$database rd ) ) as s02 from rdb$database;
select sum( ( select sum( rf.rdb$field_position ) from rdb$relation_fields rf) ) as s02 from rdb$database;

----------------------------------------------------------------------

select sum( sum( (select sum( 1 ) from rdb$database rd ) ) ) as s03 from rdb$database;
select sum( sum( (select sum( rf.rdb$field_position ) from rdb$relation_fields rf ) ) ) as s03 from rdb$database;

Last two statements will finish with this:

===
select sum( sum( (select sum( 1 ) from rdb$database rd ) ) ) as s03 from rdb$database;
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Nested aggregate and window functions are not allowed

select sum( sum( (select sum( rf.rdb$field_position ) from rdb$relation_fields rf ) ) ) as s03 from rdb$database;

S03 <null> -------------------------------- ???

Why we can see here "S03 <null>" (and it's what source query returns) ?

PS. WI-T4.0.0.680, WI-V3.0.3.32738