If you want to use page caching, but can’t because you will loose the ability to display error and succcess messages, there is a way around this.
You can put your error in javascript so your pages are always cached, only your javascript will change.
You will need to make sure javascript is loaded from the webserver and that the browser isn’t caching your javascript.
In your javascript template have some code like:

<% if flash[:js_notice]  %>

  var js_notice = "<%= flash[:js_notice]  %>"

<% else %>

  var js_notice = null

<% end %>

<% if flash[:js_error]  %>

  var js_error = "<%= flash[:js_error] %>"

<% else %>

function display_flash(){

  if (js_notice){

    var div = document.getElementById("js_notice")

    div.innerHTML =  js_notice

  }

  if (js_error){

    var div = document.getElementById("js_error")

    div.innerHTML = js_error

  }

}

window.onload = display_flash

  <div id="js_error"></div>
  <div id="js_notice"></div>