This American slang term means that it is often the small details of something which make it difficult or most challenging. These details can prolong a task, or foil an otherwise straightforward deals. Like many proverbs which involve the devil, it is meant to sound a note of caution.
This reminder is intended to encourage the scientists to check their work, and to be thorough and careful. Warning: Does a nuclear deal with Iran signify a prelude to war?
Who Got What They Wanted in the Iran Nuclear Deal http://nyti.ms/1L7kOir
BBC News - Newspaper headlines: Iran nuclear deal and 'Pluto on Pluto' http://www.bbc.com/news/blogs-the-papers-33531894
In Iran deal, Obama sees validation for diplomatic gamble: http://hosted2.ap.org/APDEFAULT/3d281c11a96b4ad082fe88aa0db04305/Article...
How the Pentagon got its way in Iran deal http://www.politico.com/story/2015/07/pentagon-iran-deal-weapon-restrict...
President Obama will hold a White House press conference later this morning. Stay tuned for live coverage.