The devil is in the details
The details of a plan, while seeming insignificant, may contain hidden problems that threaten its overall feasibility.
The source of the proverb 'The devil is in the details' is often attributed to the German/American architect Ludwig Mies Van Der Rohe. This is almost certainly a misattribution. The expression derives from an earlier German proverb - "Der liebe Gott steckt im detail", which translates as 'God is in the detail'. Mies Van Der Rohe is also associated with this earlier form but, although he may have used it, there's no evidence that he was the first to do so.
In the migration of the phrases an 's' was added - the earlier form is usually 'God is in the detail'; the later form is more commonly 'the devil is in the details'.
'The devil is in the details' only came into common use in the 1990s (Van Der Rohe died in 1969) and the earliest citation of it that I've found in print is in Richard Mayne's explanation of the workings of the European Union - The Community of Europe, 1963:
On the principle that ‘the devil is in the details’, what should have been a merely formal occasion developed into a debate about the Community's official languages and the site of its headquarters.
The phrase might have been tailor-made for negotiations between European Union countries, which are renowned for their labyrinthine and hair-splitting attention to detail.