These write ups on ACH by the zenpayroll team are fantastic. I've had to implement similar files, like settlement files, EFT files (for Canada), etc, and they are not fun, and the various rules you have to deal with when interacting with financial institutions can really boggle the mind.

One institution I was dealing with had a spec for a fixed-record-length file, but when I received the file in production, they truncated it to the last non-whitespace character, which was something not in the spec.