Back in college, I created a small desktop app that played a WAV file when any key was pressed, but could be closed silently with the mouse. (Its purpose, along with a recording of me screaming, was to train my cats not to sleep on my laptop.) It would have been written in Java 1.4 or maybe 1.3. The applet library was a pretty standard way to open and play an audio file, because it was simple.

Fast forward a few years, I came across the app again and decided to see if it still worked. It didn't work with the existing JRE on my box. So I rebuilt it with a more recent JDK (1.6, I think). It built, but it still didn't work.

See, Java 1.5 AKA "Java 2" had a new memory model, deprecated a lot of things (like applets), and started modernizing heavily. They attempted to maintain backward compatibility, giving us things like the awful erasure-based generics, but it really wasn't fully BC. They really should have just done what Python did with Python 3 and make Java 2.0 non-BC.

This isn't an isolated incident, either. A major project I worked on updated from Java 1.6 to 1.7 and it broke several things. They were fixed pretty quickly, but it still wasn't truly backwards compatible. It's just like the Java claim that you can write once and run anywhere, which is only half-jokingly referred to by devs as "write once, debug everywhere".