Syncronize before hacking into the packet list. In addition,

copy over any unprocessed packets into the new list.

When we're done, we'll also clean up properly by copying over 
any remaining packets.

ProtocolLib/src/com/comphenix/protocol/injector/PlayerInjector.java

@@ -214,7 +214,12 @@ class PlayerInjector {
 			for (Field field : list.getFields()) {
 				VolatileField overwriter = new VolatileField(field, networkManager, true);
 				
-				overwriter.setValue(Collections.synchronizedList(new ArrayList<Packet>() {
+				@SuppressWarnings("unchecked")
+				List<Packet> minecraftList = (List<Packet>) overwriter.getOldValue();
+				
+				synchronized(minecraftList) {
+					// The list we'll be inserting
+					List<Packet> hackedList = new ArrayList<Packet>() {
 						@Override
 						public boolean add(Packet packet) {
 	
@@ -245,7 +250,18 @@ class PlayerInjector {
 								throw new RuntimeException("Reverting cancelled packet failed.", e.getTargetException());
 							}
 						}
-				}));
+					};
+					
+					// Add every previously stored packet
+					for (Packet packet : minecraftList) {
+						hackedList.add(packet);
+					}
+					
+					// Don' keep stale packets around
+					minecraftList.clear();
+					overwriter.setValue(Collections.synchronizedList(hackedList));
+				}
+				
 				overridenLists.add(overwriter);
 			}
 		}
@@ -341,11 +357,29 @@ class PlayerInjector {
 		}
 	}
 	
+	@SuppressWarnings("unchecked")
 	public void cleanupAll() {
 		// Clean up
 		for (VolatileField overriden : overridenLists) {
+			List<Packet> minecraftList = (List<Packet>) overriden.getOldValue();
+			List<Packet> hacketList = (List<Packet>) overriden.getValue();
+			
+			if (minecraftList == hacketList) {
+				return;
+			}
+	
+			// Get a lock before we modify the list
+			synchronized(hacketList) {
+				try {
+					// Copy over current packets
+					for (Packet packet : (List<Packet>) overriden.getValue()) {
+						minecraftList.add(packet);
+					}
+				} finally {
 					overriden.revertValue();
 				}
+			}
+		}
 		overridenLists.clear();
 	}
 }