The selective attention test (YouTube video link) is quite well-known. If you haven't heard of it, watch it now.
Now try the sequel (another YouTube video).
Even when you're expecting the tbevyyn, you still miss other things. Attention doesn't help in noticing what you aren't looking for.
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.
July Part 1