Why is this an issue?

object.Equals() overrides can be optimized by checking first for reference equality between this and the parameter. This check can be implemented by calling object.ReferenceEquals() or base.Equals(), where base is object. However, using base.Equals() is a maintenance hazard because while it works if you extend Object directly, if you introduce a new base class that overrides Equals, it suddenly stops working.

This rule raises an issue if base.Equals() is used but base is not object.

Noncompliant code example

class Base
{
  private int baseField;

  public override bool Equals(object other)
  {
    if (base.Equals(other)) // Okay; base is object
    {
      return true;
    }

    return this.baseField == ((Base)other).baseField;
  }
}

class Derived : Base
{
  private int derivedField;

  public override bool Equals(object other)
  {
    if (base.Equals(other))  // Noncompliant
    {
      return true;
    }

    return this.derivedField == ((Derived)other).derivedField;
  }
}

Compliant solution

class Base
{
  private int baseField;

  public override bool Equals(object other)
  {
    if (object.ReferenceEquals(this, other))  // base.Equals is okay here, but object.ReferenceEquals is better
    {
      return true;
    }

    return this.baseField == ((Base)other).baseField;
  }
}

class Derived : Base
{
  private int derivedField;

  public override bool Equals(object other)
  {
    if (object.ReferenceEquals(this, other))
    {
      return true;
    }

    return base.Equals(other) && this.derivedField == ((Derived)other).derivedField;
  }
}