The resource has already committed the response

I am getting an error 500: ERROR: Cannot forward. The resource "/ssd/main/error.jsp" has already committed the response.

What exactly does this mean and how can I fix it?

Here is part of my code:

sendPage("/ssd/main/error.jsp", request, response);

Then I have a method:
protected void sendPage(String url, HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

RequestDispatcher dispatcher =
getServletContext().getRequestDispatcher(url);
dispatcher.forward(request, response);
}

Any help is greatly appreciated