[Taylor]

If a HP bowled it just right, so that the boulder hopped at the berm and vaulted over the low bar (which we have done in practice), there would be no penalty.
If a HP bowled it just right, so that the boulder struck the corner of the low bar fabric and snuck under (which we have done in practice), there would be no penalty.
Now, if the HP accidentally throws a bit too hard and it rolls through, instant foul. Because the field was faulty.

Not cool.