Tefroy Tarion's Handbook
The shear line isn't the culprit. It's necessary evil for the pin-tumbler lock, despite what they say in Adessa. Neither is master keying any more to blame than a blacksmith is for sword breaking when used as a hammer. The true problems, as Crinneus points out, are the bittings.
Without a finer method of shaping keys, we're stuck with crude assortments of pins, unable to employ the complexity and intricacy needed to foil would-be thieves. A practiced, steady hand will defeat even the most sophisticated lock.
Of course, working with such imprecise tools is a handicap for any craftsperson, but such is our plight. While we labor like brutes, the underhanded also struggle with their lockpicks. Most lockpicks will break under the slightest of duress. Some are reinforced to allow leeway, and I've even heard mention of a lockpick shaped from "prismere." I know little of the material, but it is rumored to be able to force even the strongest pin configuration.
If this is the case then it may be time to rethink the lock completely. Any fool with this sort of lockpick could open a masterfully crafted lock, perhaps even a magically reinforced one.