Attention, memory and perception
Banner blindness
People ignore anything that looks like an advert, including useful content that happens to be styled or placed like one.
Years of ads have trained people to skip boxes in the sidebar, bright banners across the top and anything that looks promotional.
Keep important content in the main flow of the page, styled like the rest of it. If a message must stand out, test that people notice it.
In practice
A help site that puts an urgent outage notice in a colourful box in the right-hand column can find that most visitors never read it.
Common mistake
Making things more colourful and animated to beat the blindness. That makes them look even more like ads.
Where it comes from
Named by Jan Panero Benway and David Lane in 1998, from a study in which people failed to find information shown in a large banner.