We've all been there: You've just spun up a new EC2 instance, you've got your key pair ready, and you're feeling good. Then you try to connect, and all you get is a "Connection timed out" error. My first instinct, and likely yours, is to look at the security group. Nine times out of ten, that's where the problem is hiding.

In this article, I'll walk you through the most common security group pitfalls that lead to EC2 connectivity issues. This isn't a comprehensive guide to every possible AWS problem, but a practical checklist focused on what a mid-level developer needs to check first.