colleague: AWS is facing some network issue.

me: But I see you are able to SSH into EC2 instances.

colleague: Well, I am running my nodejs app on 8080 but can't access it over the Internet. Works fine on my laptop though.

me: ec2_prod ~# netstat | grep node
lolol -> 127.0.0.1:8080 node

Turns out it was running on localhost IP. Worked fine on his laptop though.

last minute, obvious candidate