“The Devil is in the details” is a saying which has two completely different, but equally valid meanings — depending on whether or not you have OCD/OCPD.
If you don’t have OCD (or OCPD), and you’re frequently careless, “the Devil is in the details” because Mr. Devil is trying to mess you up by putting important things in the details, which you will clumsily overlook.
But if you do have OCD (or OCPD), you might frequently be scrupulous, and “the Devil is in the details” because Mr. Devil knows you feel doomed to keep checking over and over to make sure you got every little one of those things correct.
So the Devil really is in the details — but not always for the reason we might think.