Devil is in the detail/details
Derives from the earlier expression ‘God is in the detail/details’ meaning that finding God, goodness or perfection in some task or project requires thorough work and application. 'The devil is in the detail/details' is simply the opposite i.e. to find drawbacks or obstacles in some task or project requires the same amount of work as finding the positives. Although seemingly opposite, both expressions can amount to the same thing, in the sense that searching for both positives and negatives can lead to satisfactory end results. ‘God is in the details’ is most often attributed to Flaubert (1821-1880) but Bartlett’s Familiar Quotations lists it as anonymous. The first citation for the devil version is relatively modern, from the early 1960s.