There's one kerb around Kyoto National that seems to have a small physics glitch, I don't run in to it much, but when you run along the kerb at the right angle, the car just absolutely loses control. I've posted a small video capture to show what I mean.

http://wild.ugbox.net/dump/ripple.avi (3.6mb XviD)

if you look closely, theres a little "step" right after the kerbs like this |-|, its part if the design of the curbs so its not abused, i love that little step

I tend to loose traction when someone is right behind me and I am racing a car with downforce. Atleast I feel like that :-P

There is another strange curb on that track: at the slow right hander just after the 2nd split if we hit the inside curb sometimes it flips RAC (We just had a race with it) - the car was not sliding when going on curb. And the funny thing is that I know about 2 other people who had the same problem and every time the car has an accurate 360 degree flip... so it ends in the correct direction.