Linode's NodeBalancer assumes (as of Apr 2015) that a 500 response means that the node should be removed from rotation. Naturally, exceptions happen, so this is a very serious design limitation for any application which allows its code to have uncaught exceptions. I have opened a support ticket, with discussion copied here.
Ultimately, we had to rewrite our 500 responses to a non-50x response, which is strange to our application, but at least the change was limited to an nginx config and a single line of JavaScript to handle our status code as a server error. Linode specifically advised to use a non-50x response. All we need is a configuration in the NodeBalancer to not use passive checks on 500 Internal Server Error responses. There is no such configuration.
Due to the head-scratching nature of this configuration, we used 418 I'm a teapot
in place of 500 responses. In nginx:
proxy_intercept_errors on;
error_page 500 =418 /_error/internal-serve