Disarm and cross check gone wrong

I admittedly don’t understand the safety procedures used by the crew, and I’m interested in what was going on during a recent flight where something went wrong. This was on an A321.

After landing, the FA waived through the tiny window to the gate crew, but did not give a thumbs up. (I think this was mistake #1. She had not called for cross check and disarm, perhaps mistake #2.) The gate agent began opening the door and the FA started yelling NO and attempted to stop him. She was screaming at him to stop but the door was opening. The captain came running out of the cockpit, and yelled at her to disarm the opposite door. She was incredibly flustered and the captain was not amused.

I assume there was a risk of the slide deploying but curious if anyone can share with me what should have happened and what might have happened had she not disarmed the opposite door?

I was first to exit the plane so I had a front row seat to the excitement.