We’re all tempted to say “oh this is easy to use, it’s very straight forward”. I’ve sure done it. But the human brain is a strange and complex thing and more often than not when I test my “very straight forward” theories, there are usually big holes in it.
Jakob Neilsen posted about 86% of people who couldn’t find the single most obvious piece of information on a page. The human brain is a mysterious thing indeed, and unless you understand every aspect of the brain, then test, test, test.