Never assume that the vehicle specs that someone gives you are correct, and never assume that you remember a machine's exact specs correctly. I forgot to take a picture of a VIN tag on a Silverado, asked the customer if it was a 2500 (he said yes), and ordered all the right brake parts...except it was a 1500, which has different axles and therefore different brakes.
#diagnosis
Big ol' no shit, but...always talk to the end user of the thing that's being built or repaired about what their needs are. The person paying for the job may not be the person who knows what needs to be done, even if they think they do.