Comment 1:

IMHO: Works as intended. The attached rpctest.go is buggy
from line 53 on (a bit restructured)
  done := done := make(chan *rpc.Call, 8)
  addReply, mulReply := new(Reply), new(Reply)
  mulCall := client.Go("Arith.Mul", args, mulReply, done)
  addCall := client.Go("Arith.Add", args, addReply, done)
  addCall = <-addCall.Done
  if addReply.C != args.A+args.B {
    log.Fatalf("Add: expected %d got %d", addReply.C, args.A+args.B)
  }
So: addCall.Done == mulCal.Done == done
Now addCall = <-addCall.Done is basically
firstFinishedCall := <- done; addCall = firstFinishedCall
which might be the multiplication. 
Testing now for addReply != something if wrong as the
firstFinishedCall might not be the addition
(even when stored into a variable called "addCall").

Comment 2:

Status changed to Invalid.