Often, when critiquing a design, I ask how a designer came to make a certain decisions. Too often they reply “it’s consistant.”
Consistency is a tactic, not a goal. I know many many lists of heuristics like to canonize it as a desirable quality like beautiful and usable, but really it’s just a way to achieve a particular end result; in this case a learnable interface.
If you place interface objects in the same place, and have them always look the same across all screens, you can teach users what their purpose is. If you use the same visual indicators other interfaces use, the learning can be brought down to near nothing. For example, nearly all eCommerce sites use the shopping cart to represent everything which you have chosen to buy but for which you have not yet paid. When you sit down to design your eCommerce site, you could use a shopping bag and hope for the best, or try out a milk crate and cross your fingers, but being consistent with other sites is the quickest way to make sure users can give you money.
Being consistent in an interface also speeds up task completion. The first time you send a email you might have to hunt around for the send button, but by the third or fourth time it’s easy and soon after it’s automatic. In fact, if the buttons are always in the same location in your mail programs you can send email, delete trash and mark spam far more quickly, allowing you to focus on the art of insulting your boss without him realizing it. Move the buttons around and your users may accidently cc someone who can explain the insult to him. Inconsistancy can be trouble for a user in this scenario.
So: consistency! Obviously a great good! Why do anything else?
When your goal is not a learnable interface. When your goal is garnering attention.
For example, if every item in your navigation bar is white on black, and you want people to see a new section just added, you might make it red. Or let’s imagine you have situation where saving a file doesn’t just save it, it saves and replaces all previous drafts. Perhaps then you might want to make it yellow instead of blue and call it “save and overwrite.”
There are times when you want the user to not have to think, but there are also times when you very much want the user to sit up and pay attention. Consistency lulls us, inconsistency wakes up up.
Now you have two tools in your tool belt, instead of one virtue and one vice.