Gunicorn is an application server for Python applications that uses the Web Server Gateway Interface (WSGI) to communicate with a web server. It is often deployed behind an NGINX web server, which proxies requests and passes them on to Gunicorn worker processes. However, if Gunicorn isn't running, NGINX will return a 502 Bad Gateway error. Common causes of 502 errors include Gunicorn not being run, NGINX unable to communicate with Gunicorn, or Gunicorn timing out. To resolve these issues, it's essential to check the status of Gunicorn and ensure that its socket configuration matches the NGINX configuration. Additionally, increasing Gunicorn's timeout value can help prevent 504 Gateway Timeout errors when NGINX times out before receiving a response from Gunicorn. Collecting logs and application performance monitoring (APM) data with a log management service like Datadog can provide valuable insights into identifying bottlenecks and resolving issues such as 502 errors.