Devil is in the details

The phrase the devil is in the details means small details can cause big problems if they are ignored. Something may look simple at first, but the tricky parts are often hidden so you should pay attention to the details.
This idiom was originally "God is in the details" meaning thoroughness is Godly, but evolved to emphasise the opposite, that not caring about the details invoked the devil.

The idea sounded easy, but the devil was in the details.