This got my attention after reading Norman Dunbar's Firebird Internals document.

When more than 32K generators are created (and deleted), the generator id is restarted. As soon that happens, new generators start getting old (of already deleted) current values.

Test case:

execute block
as
  declare n integer = 0;
  declare x integer;
begin
  while (n < 33000)
  do
  begin
    in autonomous transaction do
       execute statement 'create sequence s' || n;
    in autonomous transaction do
       execute statement 'select gen_id(s' || n || ', 1000) from rdb$database' into x;
    in autonomous transaction do
       execute statement 'drop sequence s' || n;

    n = n + 1;
  end
end!

-- OK: No sequences.
show sequence!

create sequence s1!

-- Error: S1 is at 1000!
show sequence!