How a simple UI change can have drastic effects, part the first:

About 2 months ago Twitter changed the order of your Follower list. It used to list your followers, in order that they joined twitter. Now it lists the people that just followed you.

Soon the users figured out that ‘people that just followed you’ was a gold mine of people that just took the action of following someone. Open up a few profiles, follow the 10 people that just followed them. Repeat until you have 13k followers.

Part the second:

It’s hard to imagine a form that could be simpler: two fields, two buttons, and one link. Yet, it turns out this form was preventing customers from purchasing products from a major e-commerce site, to the tune of $300,000,000 a year.

Unintended consequences are something of an art form in interface design. Small things can affect behavior, not just in your user, but across an entire community. Everything from button placement, to type size, to what features are exposed where, creates an environment that either encourages or discourages certain types of behavior.

About a year ago, I did a quick consult on a forum UI for a friend. Like many communities, the forum had a simple feature to mark a given user as either a “friend,” someone allowed to message you and whose posts where marked in a special way, or an “enemy,” someone who couldn’t message you and whose posts where hidden.

His first draft exposed the feature in an utterly sane way. Two links, next to any user’s name. “Add friend,” and “Make enemy”. Fast access to a useful feature. I’m sure he didn’t give it too much thought.

My first criticism? Kill the enemy link. Hide the feature somewhere deep in the UI. Get rid of it all together if he could.

It wasn’t just that the feature existed, but it’s prominent placement made knee-jerk reactions too easy. Someone post something you don’t agree with? There’s a nice quick way to get rid of them. Over time, the community would grow increasingly negative, myopic, and exclusive. All because of one link.

This may seem non-sequitur to the examples above, but they all sit in this neat boat of thinking about more than just usability when designing.

Everything is a trade-off. Everything has a consequence.