What is a genetic modification that would cause a massive chain reaction to possibly horrific effects?

Let's keep things simple two rules.

• No giving sentience, This is a no brainer issue.
• Let's keep it to beings under the Animilia kingdom. "mutated virus/bacteria" is a common trope.

To start:

Let's modify ants to have lungs.

Most insects are constrained by the amount of oxygen they can acquire through their exoskeleton.

Imagine how big they can get if they didn't have that constraint?