Endpoint terminating, "the response was already sent" (Plug.Conn.AlreadySentError)

errors.ex

[error] #PID<0.504.0> running T.Endpoint terminated
Server: localhost:4000 (http)
Request: POST /signup
** (exit) an exception was raised:
    ** (Plug.Conn.AlreadySentError) the response was already sent
        (plug) lib/plug/conn.ex:526: Plug.Conn.put_resp_header/3
        (phoenix) lib/phoenix/controller.ex:301: Phoenix.Controller.redirect/2
        (t) web/controllers/registration_controller.ex:1: T.RegistrationController.action/2
        (t) web/controllers/registration_controller.ex:1: T.RegistrationController.phoenix_controller_pipeline/2
        (t) lib/phoenix/router.ex:255: T.Router.dispatch/2
        (t) web/router.ex:1: T.Router.do_call/2
        (t) lib/t/endpoint.ex:1: T.Endpoint.phoenix_pipeline/1
        (t) lib/plug/debugger.ex:90: T.Endpoint."call (overridable 3)"/2
        (t) lib/phoenix/endpoint/render_errors.ex:34: T.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4