app.js

helpButton.on("click", event => {
  chan.push("help_needed", {})
})

gistfile1.txt

[error] GenServer #PID<0.299.0> terminating
Last message: %Phoenix.Socket.Message{event: "help_needed", payload: %{}, ref: "2", topic: "help"}
State: %Phoenix.Socket{assigns: %{}, channel: HelpPlease.HelpChannel, channel_pid: #PID<0.299.0>, endpoint: HelpPlease.Endpoint, joined: true, pubsub_server: HelpPlease.PubSub, ref: nil, topic: "help", transport: Phoenix.Transports.WebSocket, transport_pid: #PID<0.297.0>}
** (exit) an exception was raised:
    ** (RuntimeError) Expected `handle_in/3` to return one of:

    {:noreply, Socket.t} |
    {:reply, {status :: atom, response :: map}, Socket.t} |
    {:reply, status :: atom, Socket.t} |
    {:stop, reason :: term, Socket.t} |
    {:stop, reason :: term, {status :: atom, response :: map}, Socket.t} |
    {:stop, reason :: term, status :: atom, Socket.t}

got {:no_reply, %Phoenix.Socket{assigns: %{}, channel: HelpPlease.HelpChannel, channel_pid: #PID<0.299.0>, endpoint: HelpPlease.Endpoint, joined: true, pubsub_server: HelpPlease.PubSub, ref: "2", topic: "help", transport: Phoenix.Transports.WebSocket, transport_pid: #PID<0.297.0>}}

        (phoenix) lib/phoenix/channel/server.ex:255: Phoenix.Channel.Server.handle_result/2
        (stdlib) gen_server.erl:593: :gen_server.try_dispatch/4
        (stdlib) gen_server.erl:659: :gen_server.handle_msg/5
        (stdlib) proc_lib.erl:237: :proc_lib.init_p_do_apply/3

help_channel.ex

  def handle_in("help_needed", payload, socket) do
    broadcast! socket, "help_needed", payload
    {:no_reply, socket}
  end

router.ex

socket "/ws" do
  channel "help", HelpPlease.HelpChannel
end

Old post, but someone in the irc channel found it and was hoping for a solution, so just putting an answer here :)
the correct response for help_channel.ex:3 is: {:noreply, socket} notice the missing underscore