OWASP - Java Gotchas

As I spend more time developing in Java and analysing others source code for quality and security issues I come across some common pitfalls that often catch Developers out, some which can have serious consequences on the logic. From time to time I rip open some of the Java classes developed by Sun to get a deeper understanding as to what goes on behind the scenes. There is a particular topic that I enjoy contributing to on OWASP called Java Gotchas (unfortunately not as much as I would like to). If Java is your thing I am sure you will find something on there of interest and of course please feel free to contribute.