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

@@ -214,38 +214,54 @@ class PlayerInjector {
 			for (Field field : list.getFields()) {
 				VolatileField overwriter = new VolatileField(field, networkManager, true);
 				
-				overwriter.setValue(Collections.synchronizedList(new ArrayList<Packet>() {
-					@Override
-					public boolean add(Packet packet) {
-
-						Packet result = null;
-						
-						// Check for fake packets and ignored packets
-						if (packet instanceof FakePacket) {
-							return true;
-						} else if (ignoredPackets.contains(packet)) {
-							ignoredPackets.remove(packet);
-						} else {
-							result = handlePacketRecieved(packet);
-						}
-						
-						// A NULL packet indicate cancelling
-						try {
-							if (result != null) {
-								super.add(result);
+				@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) {
+	
+							Packet result = null;
+							
+							// Check for fake packets and ignored packets
+							if (packet instanceof FakePacket) {
+								return true;
+							} else if (ignoredPackets.contains(packet)) {
+								ignoredPackets.remove(packet);
 							} else {
-								// We'll use the FakePacket marker instead of preventing the filters
-								sendServerPacket(createNegativePacket(packet), true);
+								result = handlePacketRecieved(packet);
 							}
 							
-							// Collection.add contract
-							return true;
-							
-						} catch (InvocationTargetException e) {
-							throw new RuntimeException("Reverting cancelled packet failed.", e.getTargetException());
+							// A NULL packet indicate cancelling
+							try {
+								if (result != null) {
+									super.add(result);
+								} else {
+									// We'll use the FakePacket marker instead of preventing the filters
+									sendServerPacket(createNegativePacket(packet), true);
+								}
+								
+								// Collection.add contract
+								return true;
+								
+							} catch (InvocationTargetException e) {
+								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);
 			}
 		}
@@ -341,10 +357,28 @@ class PlayerInjector {
 		}
 	}
 	
+	@SuppressWarnings("unchecked")
 	public void cleanupAll() {
 		// Clean up
 		for (VolatileField overriden : overridenLists) {
-			overriden.revertValue();
+			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();
 	}