Normal mud comes off fine. Mud from slides on the arses of jeans does not.
It's happened twice now, once on dd1's new jeans (which are beige and patterned anyway, so it's not too bad, you can't see it unless you know it's there) and once on dd2's white jeans (MIL)
With the white jeans I have even tried soaking them in a bucket of bleach but it just will not shift.