Warn instead of throwing an exception on packet type change.

We don't want to crash plugins over this, since it doesn't 
automatically cause problems. But it may trip up plugins that 
assume the packet types they set when registering a listener is the 
only ones they'll ever recieve in the method body, which is not true
if a preceeding packet listener can change a packet to an arbitrary 
type.

I'm open for better suggestions here. But for now, I'll just print a 
warning and hope people use sendServerPacket() instead.

ProtocolLib/src/main/java/com/comphenix/protocol/events/PacketEvent.java

@@ -28,11 +28,28 @@ import org.bukkit.event.Cancellable;
 
 import com.comphenix.protocol.Application;
 import com.comphenix.protocol.PacketType;
+import com.comphenix.protocol.ProtocolLibrary;
 import com.comphenix.protocol.async.AsyncMarker;
+import com.comphenix.protocol.error.Report;
+import com.comphenix.protocol.error.ReportType;
 import com.google.common.base.Objects;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.HashMultimap;
+import com.google.common.collect.Multimaps;
+import com.google.common.collect.SetMultimap;
 
+/**
+ * Represents a packet sending or receiving event. Changes to the packet will be 
+ * reflected in the final version to be sent or received. It is also possible to cancel an event.
+ * @author Kristian
+ */
 public class PacketEvent extends EventObject implements Cancellable {
+	public static final ReportType REPORT_CHANGING_PACKET_TYPE_IS_CONFUSING = new ReportType(
+			"Plugin %s changed packet type from %s to %s in packet listener. This is confusing for other plugins! (Not an error, though!)");
+	
+	private static final SetMultimap<PacketType, PacketType> CHANGE_WARNINGS = 
+			Multimaps.synchronizedSetMultimap(HashMultimap.<PacketType, PacketType>create());
+	
 	/**
 	 * Automatically generated by Eclipse.
 	 */
@@ -170,8 +187,19 @@ public class PacketEvent extends EventObject implements Cancellable {
 			throw new IllegalStateException("The packet event is read-only.");
 		if (packet == null)
 			throw new IllegalArgumentException("Cannot set packet to NULL. Use setCancelled() instead.");
-		if (this.packet != null && !Objects.equal(this.packet.getType(), packet.getType()))
-			throw new IllegalArgumentException("Cannot change packet type from " + this.packet.getType() + " to " + packet.getType());
+		
+		// Change warnings
+		final PacketType oldType = this.packet.getType();
+		final PacketType newType = packet.getType();
+		if (this.packet != null && !Objects.equal(oldType, newType)) {
+			// Only report this once
+			if (CHANGE_WARNINGS.put(oldType, newType)) {
+				ProtocolLibrary.getErrorReporter().reportWarning(this, 
+						Report.newBuilder(REPORT_CHANGING_PACKET_TYPE_IS_CONFUSING).
+						messageParam(oldType, newType).
+						build());
+			}
+		}
 		this.packet = packet;
 	}