In a situation where I'm adding a new feature to the language, with a new syntax that causes me to add a new rule to the grammar, I'm concerned that the new rule will accidentally "capture" some code that already exists in the wild, that was previously derived by another part of the grammar.

To give a very ugly example, if you have a language with function calls f(expr, expr, expr) and you want to add tuple syntax to expressions with a brand new rule:

  expr := expr COMMA expr

Then you might have accidentally turned all functions into unary functions, as the tuple rule captures the "expr, expr, expr" part and leaves you with f(expr).