If you are a web developer, you will be very familiar with this problem: browsers and web servers cache stuff to help speed up response. The idea is great when things don't change much, but it ain't rosy when pages, scripts, code, and content are changing constantly and quickly.
Most browsers offer a "refresh" button that's supposed to get the latest content. Don't be surprised if it's not always the case. I think I relied too much on that feature and it screwed me over, causing me to hack my own code before I realized it could be the browser's fault (and rightly so). I've seen instances where refresh can load an old (cached) script even when a newer version exists. Reducing cache size to mere MB helped - my first clue. Usually when I suspect this kind of issue, i switch to another browser (IE does cache better) and see if things look the same.
Web servers also cache stuff. The most dangerous are what they save when shutting down so that they can restore when they are back online. I've seen instances where old stuff was used instead of newly deployed code. Tomcat is notorious for this. I disabled this kind of caching a while back (don't recall how I did it, but suddenly it was gone as I tweaked around).
Compilers are the third and final source of caching issues. Most advanced IDEs have a "clean project" feature that helps. In a nutshell, if your files didn't change, some compilers won't recompile them when you build the project - even when dependencies may have changed. The result is a product with mixed code, old and new together. The surest sign of this is new features with old data - what I call "history playback".