I am receiving traps by implementing SnmpClient (which means writing the methods for authenticate, debugPrint, and callback).  It works fine for v2 traps and some v3 traps.  From a particular device however, when a v3 trap comes in, the 'authenticate' method does not get executed, and the PDU in the callback is null.  I see the trap come in and get decoded using wireshark, and I was able to turn on debugging and see the trap data (in hex) be written to the log file in the debugPrint method in the receiver.  I also (sometimes) see an error in the log: 'Error decoding Packet: Parse Auth: Incorrect VarBind'.