Calling addNotify on a component at the wrong time can cause the java runtime to crash sometime later. As far as I've been able to track down, it happens like this: if addNotify gets called for a component twice, the peer created the first time is discarded but not destroyed. The X window for the peer still exists and the Xt event handler is still set. If, then, the discarded peer gets garbage-collected, if an event happens for the peer's X window, the event handler tries to act on a null object and dies with a segmentation fault.