Sign in possibly succeeds for null user

[robingram]

I wonder if you might have any insight into a strange error that we're getting.

We've been receiving occasional exceptions (a few per week) from our sessions controller where the result of sign_in is success but the user record is nil. Our create action is pretty standard:

def create
  @user = authenticate(params)

  sign_in(@user) do |status|
    if status.success?
      after_login_actions
      redirect_back_or url_after_create
    else
      flash.now.alert = error_message
      render :new, status: :unauthorized
    end
  end
end

after_login_actions is really simple but the first thing it does is call a method on @user, and this is where were getting the errors because @user is nil.

We don't have any custom sign in guards so we're a little confused about why sign_in is either succeeding for an empty user record or blanking it itself somehow.

Any suggestions on what might be causing this or steps to diagnose what's happening?