What does 'devil in the details' mean?

[Priya Raghavan]

English Language Tests, Intermediate level

ESL/EFL Test #332 "English Slang Idioms (53)", question 6

"I know you aren't that worried about mistakes on the forms but the devil's in the .......... These mistakes can cost our customers a lot of money," the boss told the customer service team.

(a) errors
(b) paper
(c) attitude
(d) details

English Language Tests, Intermediate level

ESL/EFL Test #332 "English Slang Idioms (53)", answer 6

"I know you aren't that worried about mistakes on the forms but the devil's in the details. These mistakes can cost our customers a lot of money," the boss told the customer service team.

Correct answer: (d) details
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What does 'devil in the details' mean?

Priya Raghavan

[Daemon99]

Priya,

I suggest you use google more often. She knows everything

http://www.usingenglish.com/reference/idioms/devil+is+in+the+detail.html
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Non-native speaker of English
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I intend to live forever - so far, so good.