I've been noticing it a lot recently that when I stand around on a hill which should only be grade 5-10 talking with someone I'll slowly slide down the hill even though I shouldn't be moving at all. The higher the grade, the faster I slide. Obviously anything higher than a 45 degree angle should be impossible to stand on without sliding, but something below 20 is easy to stay in place on. You might want to turn up the avatar friction a little.