Scenario:

1: The AI lands a unit from a full transport next to your city.
2: You establish contact for the first time.
3: You sign a peace treaty, and they AI says that they'll withdraw their units from your territory.
4: They fully unload their transport on your territory JUST AFTER THE NEGOTIATIONS ARE OVER!

Pretty annoying. Has this happened to anyone of you before? (Or a lot of times?)

Happens all the time. After all, it is the ai and can cheat. Actually, you can do the same thing. IIRC, I have made contact with the ai, signed a treaty and your units are withdrawn at that moment, but after ending the contact, I moved troops back into the ai's area.

Doing that damages your reputation, of course... but if the AI violates your territory right after a negotiation, and then you activate cheat mode and switch to playing as that civ, you will find your reputation spotless. It just isn't fair.