*Try 2: Synchronized stack alteration.

src/main/java/forge/game/zone/MagicStack.java

@@ -559,7 +559,10 @@ public class MagicStack extends MyObservable {
         }
 
         final SpellAbilityStackInstance si = new SpellAbilityStackInstance(sp);
+        
+        synchronized (this.stack) {
             this.getStack().push(si);
+        }
 
         // 2012-07-21 the following comparison needs to move below the pushes but somehow screws up priority
         // When it's down there. That makes absolutely no sense to me, so i'm putting it back for now
@@ -844,10 +847,13 @@ public class MagicStack extends MyObservable {
      * @return a {@link forge.card.spellability.SpellAbility} object.
      */
     public final SpellAbility pop() {
+        synchronized(this.stack)
+        {
             final SpellAbilityStackInstance si = this.getStack().pop();
             final SpellAbility sp = si.getSpellAbility();
             return sp;
         }
+    }
 
     public final SpellAbility top() {
         final SpellAbilityStackInstance si = this.getStack().peek();