The only real moment I noticed the AI in Halo 2 was by the giant walking thing, I think the first time you encounter it in the ancient Halo level, it's the part where you come out of some cave-type thing and there's a Jackal sniping right at the exit. Anyways, my Warthog driver had died, I only had one marine left off in the distance to my right and so I had to lay low for a short while. Well, when I thought it'd be safe to come out all guns blazing to take out the remaining Elites I noticed that a Grunt was looking for me on the ground, one Jackal was still holding his sniper position and the remaining Elites were in my Warthog, one of them in the driver seat and the other on the Guass Cannon, I guess patrolling the area.
That was the first time one of the enemies had actually taken my vehicle in the game so I was stunned. It wasn't like they were taking the vehicle away from me because it posed a threat to them, more like they took it because they could use it to pose a threat to me. I stood there for a second just trying to comprehend what was happening and got sniped in the back a second later.
Every other time in the game the AI seemed either real enough that I simply feared for my virtual life and didn't even think about it, or programmed enough that I took advantage of their stupidity and grabbed an easy kill. That one time in the Warthog, though, they actually felt intelligent. O_O