From Neal Gafter:

The following demonstrates a bug in javac's handling of wildcard types that results in a hole in the type system. javac accepts this program without even a warning but it blows up with a class cast error at runtime.  The Java compiler for Eclipse rejects this code.

import java.util.*;

class Comp {
  static <T> Comparator<T> compound(final Iterable<? extends Comparator<? super T>> it) {
    return new Comparator<T>() {
      public int compare(T t1, T t2) {
        for (Comparator<? super T> c : it) {
          int r = c.compare(t1, t2);
          if (r != 0) return r;
        }
        return 0;
      }
    };
  }
  public static void main(String[] args) {
    List<Comparator<?>> x = new ArrayList<Comparator<?>>();
    Comparator<Integer> c1 = new Comparator<Integer>() {
      public int compare(Integer i1, Integer i2) {
        return i1.compareTo(i2);
      }
    };
    x.add(c1);
    x.add(String.CASE_INSENSITIVE_ORDER);
    Comparator<String> c3 = compound(x);
    c3.compare("foo", "bar");
  }
}