Error prevention is one of the most important and frequently bungled design heuristics.

Above: good error prevention in Reddit’s registration interface. Rather than surprising the user with an error when he tries to submit an invalid form, Reddit tells the user up-front what is wrong with the inputs. This may not seem like a huge deal, but it saves time and keeps the user from becoming needlessly frustrated.

Above: a failure of error prevention in a climbing gym. The sport of indoor rock climbing is based on using only particular holds to get to the top, following a predetermined route. The routes at the pictured gym are color-coded. There is no recourse for color-blind people. At the far right of the photo, notice a red route and a green route crossing each other. Red-green color-blindness is very common, and people afflicted with it would have a lot of trouble climbing either one of those routes.

Above: a potential redesign of the climbing wall that would help color-blind users. This is basic redundant encoding of information. Conveying meaning through pattern in addition to color prevents errors for color-blind users.