在Java中遇到的最常见的并发问题是什么?

This is a poll of sorts about common concurrency problems in Java. An example might be the classic deadlock or race condition or perhaps EDT threading bugs in Swing. I'm interested both in a breadth of possible issues but also in what issues are most common. So, please leave one specific answer of a Java concurrency bug per comment and vote up if you see one you've encountered.This is a poll of sorts about common concurrenc