Limb length discrepancy, a condition where kids have one leg shorter than the other, often starts out very subtly. But more dramatic differences, an inch or more, can cause a significant limp and throw off the body's dynamics.
Razor blades in apples? Poisoned candy? Probably not going to happen. But you still need to think about costume and pedestrian safety, as well as when your kids are old enough to trick-or-treat on their own.