Overspec

In many notations, there are various ways to achieve the same effect, and information in those should not be duplicated or contradictory, since it only confuses grammar engineers and leads to grammars with very subtle bugs.

Reference

Mats Stijlaart and Vadim Zaytsev. 2017. Towards a taxonomy of grammar smells. In Proceedings of the 10th ACM SIGPLAN International Conference on Software Language Engineering (SLE 2017). ACM, New York, NY, USA, 43-54.