One of the most common causes of sporadic bugs in swing applications is doing things on the wrong thread. Most common of these is when a thread that is not the Event Dispatch Thread does something that updates the gui. Its very easy to do accidentally as seemingly innocent operations done on a background thread can fire off event listeners and end up inside code it shouldn’t as a result.
CheckThreadViolationRepaintManager from the SwingHelper project is a very useful class that can be easily plumbed into a Swing application to report any wayward threads getting into gui code.
Also from the same stable is the EventDispatchThreadHangMonitor which can report when the Event Dispatch Thread spends too long outside its main loop (which will result in a sluggish and unresponsive gui).