When people say "the devil is in the details," what do you think they mean by it?

I think that it means that the details can trip you up.

Like @marinelife to me it means while on the surface something may seem transparent and clear, not paying close enough attention to the details can result in you missing important information that may cause you problems later.

I think it means the difficulty of the situation is in the details.

To me it would depends on what they are referring to, if it were a cocktail dress, I reckon that it would be pretty naughty! If it were a contract, I would never sign, and if it were a home I was buying I’d make sure to check the foundations!

I agree with @marinelife, but it’s an expression I hear “mis-used” (if I’m allowed to say that under the rules given in your details) often. So yes, it means sometimes having to ask for clarification for an expression that should already be clear.

I see it as @marinelife said, basically. When working on a project sometimes it seems like you are almost done and everything is on schedule. Then some minor element will hold up everything either waiting for an approval, or waiting for a component you need to finish, or some sort of little technical problem you didn’t foresee. That one little detail could derail the whole project or make it late. That’s where the idea of the devil comes in!

It means that it is one thing to establish principles about what you want to do. It is another thing to make a plan to do something. And it is quite another thing to actually attempt to carry out the plan, since it will never be carried out as planned.

It is those details about starting to do something and then modifying the details of the plan in response to the real world where the devil lies. The devil never lets things go as planned. Nothing is ever smooth.

And for that reason, I rarely plan much. I have a vague intention and then I see what the devil throws my way. Then I respond. Nothing ever goes as planned, but if you don’t have a plan, then everything goes according to plan.

I understand it to refer to an effort or goal that seems easy overall, but is very difficult to achieve because of the complexity of minor elements.
For example, balancing the State budget.

How I generally hear it used is like, “Yes, on the surface that sounds great. But if you dig deeper, you’ll see that it really isn’t.” So I guess that’s what I think they mean… that there is something bad hiding in the fine print, basically.

Something along the lines of the forest but for the trees.

Nitpicking with the details and missing the bigger picture.

However, we are all familiar with God is in the details.

Read the small print on any binding document, therein lies the “sting.”