Case in Point
Caching is a very hot topic these days. Aggregation of scripts and style sheets, compression, setting Expires and Last-Modified headers. All of that is about improving loading times and reducing the amount of traffic. It's pretty smart stuff and it works pretty well.
There are always two cases to consider: a user with a non-primed cache and a user with a primed cache. If the cache is primed, the user will need to download far less files, because most of it is already cached. Scripts are the same, style sheets are the same, lots (or even all) images are the same, and maybe even the document itself is the same. Well, that's the deal basically.
Yesterday I noticed something odd, however. I visited some page I visited just the day before and all those images were reloaded. I also observed this on a lot of other websites I visit regularly. Of course I checked the headers and everything looked fine. Pretty odd. Especially if you consider that I already ramped up the cache size to 150mb ages ago.
