title: What is the most frequent concurrency issue you've encountered in Java?
body: <p>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.</p>