I have asked some version of "walk me through why this EC2 instance can't reach the internet" in about forty interviews. Maybe six people answered it well.
Not because it's hard. Because most candidates answer the question they wish had been asked. They recite what a NAT gateway is. The question was diagnostic, and diagnosis is a different skill from definition.
These are the twelve that come up most, with the reasoning behind each. Worth knowing before you read on: across 1,393 interview sessions configured on LastRound AI between January 2025 and July 2026, DevOps Engineering accounted for 464 of them. That's more than backend, frontend and full-stack combined, so the competition for these roles is real and the bar has moved.
1. Why can't you resize a VPC CIDR block after creation?
You can't shrink it. You can add secondary CIDR blocks, which is what most people mean when they say "resize."
