There is a strict system in place for violating the rules:

https://www.nacha.org/NSfines

But if things occur on accident, there is a "Returns" process for handling it. It looks like the author intends to cover that, at least partially, in the next installment. Here is a list of return codes if you want a hint:

http://www.mainstreetmerchantsolutions.com/ach-return-codes