People are very easy to confuse. A few too many options, an illogical order of items to pick from, a process that deviates from the cultural or generational norm - any of these things will quickly lead to frustration, annoyance, complaints and general negativity. Not confusing the user is critical.
This is why involvement from the client stakeholder in the design process is so important. The client is the best placed person to know the needs and wants of their users. This is also why 'design patterns' represent a dangerous shift in the industry - the implied assertion is that there is 'one true way' to design an interface that will work for everyone. That is simply untrue. Your interface has to be designed for the user, and, where possible, it needs to react to the user.
Some confusing ideas in interface building are incredibly common, and not limited to small business or individuals websites. Even the largest corporations can fail to build simple interactive experiences.
Google+ is a good example of user confusion by design. When G+ launched it came with a slew of new terms for things that users knew by other words. What everyone knows as 'chat' was called a 'hangout', grouping your friends wasn't known as 'groups' but instead was called 'circles', and so on. Google's idea was, presumably, to take a term and rebrand it as their own. The experiment has failed.
Naming design can work brilliantly. Since the launch of Twitter we have had a raft of new internet jargon added to the global lexicon - hashtags, tweets, retweeting. The notable difference however is that these terms weren't invented by Twitter, they were invented and widely adopted by the users.