As Ron stated, Spec Numbers are limited to 7 digits, giving you a maximum of 9999999 (9,999,999). If you set your startingblocknumber to 99999999 (99 million+). as expected, this will stack trace. as the startingblocknumber is 5000000 by default, this gives you 4,999,999 specs you can enter into the system.
Can you explain the purpose of changing the starting block to 99 million?