I got this message in my server log, said I should report it, so I did:

2011-12-29 19:26:17 [SEVERE] [VANISH] A flaw has been caught. Please report this (it wont break things) :) .
java.lang.ClassCastException: net.minecraft.server.Packet50PreChunk cannot be cast to net.minecraft.server.Packet32EntityLook
at to.joe.vanish.sniffers.Sniffer32EntityLook.checkPakkit(Sniffer32EntityLook.java:19)
at to.joe.vanish.sniffers.Sniffer.checkPacket(Sniffer.java:20)
at org.getspout.spout.packet.listener.PacketListeners.canSend(PacketListeners.java:70)
at org.getspout.spout.SpoutNetServerHandler.syncedSendPacket(SpoutNetServerHandler.java:584)
at org.getspout.spout.SpoutNetServerHandler.syncFlushPacketQueue(SpoutNetServerHandler.java:575)
at org.getspout.spout.SpoutNetServerHandler.queueOutputPacket(SpoutNetServerHandler.java:534)
at org.getspout.spout.SpoutNetServerHandler.sendPacket(SpoutNetServerHandler.java:519)
at net.minecraft.server.EntityTrackerEntry.a(EntityTrackerEntry.java:162)
at net.minecraft.server.EntityTrackerEntry.track(EntityTrackerEntry.java:108)
at net.minecraft.server.EntityTracker.updatePlayers(EntityTracker.java:130)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:532)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:425)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:457)

I use PEX 1.17, Spout 644, Dynmap 0.27 and CraftBukkit 1597.

Thanks for reporting. This one's been solved by some spout folks and doesn't break anything in vanishnopacket.