Why is this an issue?

A non-serializable Comparator can prevent an otherwise-Serializable ordered collection from being serializable. Since the overhead to make a Comparator serializable is usually low, doing so can be considered good defensive programming.

Noncompliant code example

public class FruitComparator implements Comparator<Fruit> {  // Noncompliant
  int compare(Fruit f1, Fruit f2) {...}
  boolean equals(Object obj) {...}
}

Compliant solution

public class FruitComparator implements Comparator<Fruit>, Serializable {
  private static final long serialVersionUID = 1;

  int compare(Fruit f1, Fruit f2) {...}
  boolean equals(Object obj) {...}
}