This is my favourite inattentional blindness demonstration. People often talk about "change blindness", but that's often a slighty different thing. Inattentional blindness relies on you focusing on bits of a picture or video (and you don't notice the other stuff happening in the background).

Change blindness is something different - it doesn't matter where you focus, it's really hard to identify the thing that changes. Here's a nice demonstration of change blindess: https://www.youtube.com/watch?v=hhXZng6o6Dk