I searched for this and didn't find anything so don't sue me if this has already been done.
Anyway, I've done a similar thread in GT3 and I thought I should start one here.
What are some stupid AI moments that you've seen (besides all the time, LOL) in GT4?
I've got a couple:
In Fuji Speedway '80s, if you do Arcade Mode in an under 30-horsepower car, something funny happens. You know the place right after the first turn where the road splits off into the chicane for the '90s version? When all the AI cars (remembering that they are 17 hp cars) go through this section, they brake! They're going like 60 and then they brake to 40. Then they start to speed up again. I don't know why.
Another thing in Fuji '80s is if you're in a midrange 250-hp ish car, like the Beetle RSi (only one that came to mind), and you come to that same place in the course, if you push another opponent into the grass they do something really stupid but really funny. They'll brake, stop, go backwards, turn, go forward like 2 feet, brake again, go backwards, turn the other way, and continue in that pattern. You can't see it during the race because you keep driving, but if you watch the reply (with the display on) in the other car's view you can see it. Eventually they leave the section but it takes like a whole lap. I'd show a replay but I don't have the right equipment, so go try it out!
One thing I'm sure you've noticed is when you get into a really fast supercar like the Tommi Kaira ZZ-II and race on Sarthe II (the one without chicanes), the AI always overshoot the turn at the end of the long straight. Normally they wouldn't, but since they're going so fast and they have so bad brakes, they brake but fly past the turn, through the 100 feet or so of sand and smack into the wall. It's really funny when all 5 of the AI's are doing it at the same time.
Finally, if you get in a Mazda RX-7 or some other Japanese sports car and go onto Suzuka Circuit (full), the AI's programming is even worse than usual. After you pass the midway point between the West and East sections, and there's the right hand turn leading into the small straight where you normally only have to go to fourth gear, the AI doesn't brake the first lap around. So, you guessed it, all the AI overshoot the turn and slide into the sand. Usually there's a couple of crashes involving spun-out cars. On the second time around, though, the light goes on and they're like, "oh! I need to brake!" so they don't do it anymore.
Post your stupid AI moments now!