In a normal C# console program, I would get the following error if I attempt to deserialize an object that isn't marked as a MessagePackObject.

MessagePack.FormatterNotRegisteredException occurred
HResult=0x80131500
Message=Messaging.CreateGameMsg is not registered in this resolver. resolver:StandardResolver
Source=MessagePack
StackTrace:
at MessagePack.FormatterResolverExtensions.GetFormatterWithVerify[T](IFormatterResolver resolver)
at MessagePack.MessagePackSerializer.Deserialize[T](Byte[] bytes, IFormatterResolver resolver)
at MessagePack.MessagePackSerializer.Deserialize[T](Byte[] bytes)

However, if this happens within Unity, it dies silently.