Solved
/serveStaleOnError doesn't work for 502 and 504
According to How to serve stale content in case no render is available I have configured /serveStaleOnError on our dispatcher(apache 2.2 dispathcer 4.2.3/4.2.2).
For testing purpose I've created the following page:
<html>
<head>
<title>Throwing an Exception</title>
</head>
<body>
<title>Throwing an Exception</title>
<%
response.sendError(503, "test failure" );
%>
</body>
</html>
serveStaleOnError works fine for 503:
And doesn't work for 504,502:
What am I doing wrong? Should anything else be configured?