Reproducable: WorldSession::Update ByteBufferException occured

Causing core freeze, lag also seen in-game

2014-09-06 12:18:01 ERROR:WorldSession::Update ByteBufferException occured while parsing a packet (opcode: 231) from client 0.0.0.0, accountid=1.
2014-09-06 12:18:01 CMSG_MOVE_WATER_WALK_ACK
2014-09-06 12:18:06 ERROR:Attempted to get in ByteBuffer (pos: 48 size: 48) value with size: 4
TargetedMovementGeneratorMedium >::unitSpeedChanged()+0x87c3 [0x0]
Unit::ProcDamageAndSpellFor()+0x67a [0x0]
WorldSession::HandleMoveWaterWalkAck()+0x53 [0x0]
WorldSession::ExecuteOpcode()+0x38 [0x0]
WorldSession::Update()+0x11c [0x0]
Map::Update()+0xa9 [0x0]
GetScriptName()+0x8709 [0x0]
CreatureAI::SummonedMovementInform()+0x398 [0x0]
BarGoLink::operator=()+0x4c1f [0x0]
ACE_OS_Thread_Adapter::invoke()+0x7f [0x0]
_get_flsindex()+0x6f [0x0]
_get_flsindex()+0x53 [0x0]
BaseThreadInitThunk()+0x12 [0x0]
RtlInitializeExceptionChain()+0x63 [0x0]
RtlInitializeExceptionChain()+0x36 [0x0]


2014-09-06 12:18:06 ERROR:WorldSession::Update ByteBufferException occured while parsing a packet (opcode: 720) from client 0.0.0.0, accountid=1.
2014-09-06 12:18:06 MSG_MOVE_TELEPORT_ACK
2014-09-06 12:18:06 Guid: Player Madtaurdruid (Guid: 31)
2014-09-06 12:18:06 Counter 0, time 12380
2014-09-06 12:18:06 Sending SMSG_INIT_WORLD_STATES to Map:0, Zone: 12

Steps to reproduce:
[LIST=1]
[*]Login as horde druid (I tested repeatedly with a tauren druid)
[*]Teleport to stormwind and jump in the canal next to the closed off stocks instance
[*]Go into Aquatic Form
[*]Swim past the guards standing on the steps just above the water line and they will agro
[*]You die at this point
[/LIST]

When dieing there is a pause as the core freezes and spits out a lot of red text regarding the WorldSession::Update

Screenshot of console error attached below.