HashMap is not thread safe. The existing synchronzed(map) is too small, should be extended to cover the surrounding code.

Mage.Client/src/main/java/mage/client/util/gui/ArrowBuilder.java

@@ -112,10 +112,11 @@ public class ArrowBuilder {
      * Removes all arrows from the screen.
      */
     public void removeAllArrows(UUID gameId) {
+        synchronized (map) {
             if (map.containsKey(gameId)) {
                 Map<Type, List<Arrow>> innerMap = map.get(gameId);
                 JPanel p = getArrowsPanel(gameId);
-            synchronized (map) {
+
                 if (p != null && p.getComponentCount() > 0) {
                     p.removeAll();
                     p.revalidate();