I happen to work with ACH/NACHA files and send them to/from various locations. The file spec outlines that there are "control" records with various values that are supposed to be used to validate the batches in the file and the file itself to prove the file is internally consistent. I have often found that when receiving the file, these control records are hard-coded and invalid. And when sending files, some vendors don't even bother to look at the control records. It's a whole different world.
Depends on who you are working with, I worked with a Bank once that gave me crappy fails and it failed because I had a validator. The new bank I work with follows the specs strictly. So if you are getting invalid files, demand that the originator of that file fix it.
