Related smells: Dependency Cycles between Packages/Subsystems, Cyclically-dependent Modularization, Static Cycles in Dependency Graphs
If nonterminals are mutually, say, left recursive, and have no non-recursive alternatives, they are useless and cannot express a proper syntactic commitment. However, a similar issue may be encountered on the level of modules, and it is harder to detect for a human because modular grammars are already stretching comprehension capabilities of a grammar engineer. Such circular dependencies are fairly easy to detect automatically with a tool.