1. the small individual parts or details of something may cause the most problems or difficulties
when negotiating, the devil is in the detail
This idiom is used to express that even seemingly small or irrelevant details of something, such as a plan, task, or design if overlooked, can result in unforeseen problems or difficulties in the future. These negative repercussions can be avoided if someone were to take the time or make the extra effort necessary to examine all aspects of something. The expression can be used in either a social or professional context but is much more common in the latter. A similar expression is 'the fine print,' or its alternative form, 'the small print.'