fixes #216

src/main/java/teetime/framework/ThreadService.java

@@ -137,10 +137,12 @@ class ThreadService extends AbstractService<ThreadService> {
 	}
 
 	private void sendStartingSignal(final Set<Stage> newThreadableStages) {
+		synchronized (newThreadableStages) {
 			for (Stage stage : newThreadableStages) {
 				((TeeTimeThread) stage.getOwningThread()).sendStartingSignal();
 			}
 		}
+	}
 
 	@Override
 	void onExecute() {
@@ -149,10 +151,16 @@ class ThreadService extends AbstractService<ThreadService> {
 
 	@Override
 	void onTerminate() {
-		for (Stage stage : threadableStages) {
+		abortStages(threadableStages);
+	}
+
+	private void abortStages(final Set<Stage> currentTreadableStages) {
+		synchronized (currentTreadableStages) {
+			for (Stage stage : currentTreadableStages) {
 				stage.terminate();
 			}
 		}
+	}
 
 	@Override
 	void onFinish() {