When a production element in the tail of an alternative is optional, some ambiguities may go undetected. This is due to not properly comparing the optional element with the succeeding tokens to the alternative. An ambiguious grammar like the one below illustrates this:

A = B ["x"] ;
B = "y" ["x"] ;

In cases where the ambiguity is resolvable this may also lead to parse errors as the wrong alternative may be selected (too few look-ahead tokens calculated).