I was testing my character setup, running around on a terrain, and decided terrain needed some more variation. So I pressed stop, got back into the editor, and starting adding some noise with one of the terrain brushes. Then, I press play again, and run out into the terrain, and my character is running up and down hills that don't exist - it's like it randomly selected some other portion of the terrain to use for collision detection. Pressed stop, checked everything out.. thought - must be a fluke - pressed play, and it did it again exactly the same. I had to exit Unity and reload it to make terrain physics work correctly. No big deal, but I thought that was really strange.