DefaultGroovyMethods.minus(List, Collection removeMe) and DefaultGroovyMethods.NumberAwareComparator have two problems.<br> 1. dupplicate values that are not Comparable cause IllegalStateExceptions <code> def a = new Object() // throws IllegalStateException assert [] == [ a ] - [ a, a ] </code><br> 2. null values are not removed from a List <code> // fails assert [] == [ null ] - [ null ] </code><br> I have attached a unit test and a fix based on the groovy-1.1-beta-2 source.