The "pilot error" is in not testing the solution before trusting it and in making an assumption that the data would be safe in an untested external system.
Let me put it into a financial context. I did a project that added up to some $800K in cost. Two developers over about a year. Do you think that for even a microsecond I would trust Dropbox as a backup mechanism without extensive qualification and testing? And, would I have that as my sole approach to backup?
Nope.
And this isn't me coming down on Dropbox or suggesting that they are unreliable. I use Dropbox and I am very happy with the service. However, based on experience gotten the hard way, I do not use it for anything that is mission critical. The important stuff is backed-up locally, sometimes with redundant backups.
I don't have any sympathy for an engineer who uses a service and relies on it without the proper testing and qualification phase. Behaving that way is most-definitely, as I said, to be kind, "pilot error".
It is completely unfair to blame Dropbox for anything. This is like the idea of cutting-and-pasting code from the Internet and trusting it with a mission-critical aspect of your application without dissecting the code, testing it and fully qualifying it for what it is you need it to do.
There are tons of examples of this behavior, perhaps the most common one in the web development community is email validity verification. How many careless developers copy a regex like this to validate their email and don't think twice about it:
^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$
These things are all over the web. And most of them suck. Even some of the most impressive ones will lead you astray.
How many engineers (and, as a result, websites) fail to test and research and end-up with questionable solutions?
Would you blame the regex writer for this or the engineer who chose to use it without doing any testing at all?
It doesn't matter what the regex author claims or how authoritative the website featuring the regex might be, you have to test it before deploying it! If you don't test, deploy and then loose customers because it is a bad regex it is YOUR fault, not the author's. Blaming them is nonsensical.
In case you are curious, here's "the" solution:
http://isemail.info/about
Not only is it well documented, the author offers test vectors and all of the relevant code and references so that you can take the time to qualify the solution.
So, yes, to be redundant, if an engineer does not do his job I am perfectly happy calling it "pilot error" and even going as far as calling it "incompetence".