Hello,
I've experimented with Firebird and I wonder, if this could be a bug:

create table temp_table ( a integer);
insert into temp_table values(1);
insert into temp_table values(2);
insert into temp_table values(3);
insert into temp_table select a from temp_table;

After the last statement server hangs in endless loop and DB grows all
the time.

I have tested this on FireBird 1.5.2., and I wonder if something
similar could happen in some complex inserts.