We're running a web app on Tomcat 6 and Apache mod_proxy 2.2.3. Seeing a lot of 502 errors like this:
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /the/page.do.
Reason: Error reading from remote server
If you think this is a server error, please contact the webmaster.
So, answering my own question here. We ultimately determined that we were seeing 502 and 503 errors in the load balancer due to Tomcat threads timing out. In the short term we increased the timeout. In the longer term, we fixed the app problems that were causing the timeouts in the first place. Why Tomcat timeouts were being perceived as 502 and 503 errors at the load balancer is still a bit of a mystery.