The typical cases are usually too simple and students fall into a sense of false security. Rather than truly mastering the skill, students develop pattern recognition that gets them through the common cases. Eliminate this problem by seeking edge cases.
Looking for some ways to challenge your advanced mathematicians? If you’d like to keep them on the same topic as the rest of your class, consider increasing the complexity of your current unit. If they’re in need of more advanced curriculum to keep their creativity flowing, try to bring in novel ways of looking at math.