The first one that comes to mind is the assumption that if it's encrypted, then it's secure. I think this is a...
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dangerous mindset. You know, encryption doesn't automatically mean security. It's all in how the encryption is implemented and managed. That's the big one, and I tend to see that most with management and less technical people. They hear that they've got laptop encryption or that they've got a hardened data center and they assume that all is well, but that's hardly ever the case. You actually have to validate that with tools and ethical hacking techniques.
Also, in most networks that I'm seeing, people don't have a clue as to what information they actually have. They also don't have a clue where it's stored or what security risks it's up against. So, if they don't know this stuff, they can't reasonably know what needs to be protected. I think the assumption is that everything that is sensitive is on servers or protected storage so there's not that much to worry about. This is wrong. I'm seeing vulnerabilities in these areas all the time. In any given network today, there are megabytes, if not gigabytes, of files scattered all across different hard drives and storage systems within the network. I think sensitive information is everywhere across the network, it's unprotected, and it's waiting to be compromised. And, a lot of people are oblivious to it in many cases.
Check out the entire Storage Encryption FAQ guide.