After making a major update to a website, the staffperson who had requested the change told me the website looked no different than before.  I was puzzled by this, because I had tested the website in 4 or 5 different web browser versions to make sure the website looked right & behaved correctly.  But as soon as I started talking with her, I suspected the problem was simply that her web browser had "cached" the old version, and wasn't checking the web server for a more recent version.