Spent three days banging my head against my desk trying to get an AWS Lambda function to work, only to finally discover that my code was perfectly functional and it was a security group problem. It was supposed to send a POST request to a load balancer's URL but couldn't resolve the hostname because the security group blocked a necessary outbound port for DNS requests.

That's what I get for not troubleshooting at the infrastructure level when experiencing connection issues. I did not spend two years doing tech support just to forget basic troubleshooting steps now that I'm in the DevOps field...