bool Document::CanAcceptChild(const Node& new_child,
                              const Node* next,
                              const Node* old_child,
                              ExceptionState& exception_state) const {
  DCHECK(!(next && old_child));
  if (old_child && old_child->getNodeType() == new_child.getNodeType())
    return true;

  int num_doctypes = 0;
  int num_elements = 0;
  bool has_doctype_after_reference_node = false;
  bool has_element_after_reference_node = false;

  bool saw_reference_node = false;
  for (Node& child : NodeTraversal::ChildrenOf(*this)) {
    if (old_child && *old_child == child) {
      saw_reference_node = true;
      continue;
    }
    if (&child == next)
      saw_reference_node = true;

    switch (child.getNodeType()) {
      case kDocumentTypeNode:
        num_doctypes++;
        has_doctype_after_reference_node = saw_reference_node;
        break;
      case kElementNode:
        num_elements++;
        has_element_after_reference_node = saw_reference_node;
        break;
      default:
        break;
    }
  }

  if (new_child.IsDocumentFragment()) {
    for (Node& child :
         NodeTraversal::ChildrenOf(ToDocumentFragment(new_child))) {
      switch (child.getNodeType()) {
        case kAttributeNode:
        case kCdataSectionNode:
        case kDocumentFragmentNode:
        case kDocumentNode:
        case kTextNode:
          exception_state.ThrowDOMException(
              DOMExceptionCode::kHierarchyRequestError,
              "Nodes of type '" + new_child.nodeName() +
                  "' may not be inserted inside nodes of type '#document'.");
          return false;
        case kCommentNode:
        case kProcessingInstructionNode:
          break;
        case kDocumentTypeNode:
          num_doctypes++;
          break;
        case kElementNode:
          num_elements++;
          if (has_doctype_after_reference_node) {
            exception_state.ThrowDOMException(
                DOMExceptionCode::kHierarchyRequestError,
                "Can't insert an element before a doctype.");
            return false;
          }
          break;
      }
    }
  } else {
    switch (new_child.getNodeType()) {
      case kAttributeNode:
      case kCdataSectionNode:
      case kDocumentFragmentNode:
      case kDocumentNode:
      case kTextNode:
        exception_state.ThrowDOMException(
            DOMExceptionCode::kHierarchyRequestError,
            "Nodes of type '" + new_child.nodeName() +
                "' may not be inserted inside nodes of type '#document'.");
        return false;
      case kCommentNode:
      case kProcessingInstructionNode:
        return true;
      case kDocumentTypeNode:
        num_doctypes++;
        if (num_elements > 0 && !has_element_after_reference_node) {
          exception_state.ThrowDOMException(
              DOMExceptionCode::kHierarchyRequestError,
              "Can't insert a doctype before the root element.");
          return false;
        }
        break;
      case kElementNode:
        num_elements++;
        if (has_doctype_after_reference_node) {
          exception_state.ThrowDOMException(
              DOMExceptionCode::kHierarchyRequestError,
              "Can't insert an element before a doctype.");
          return false;
        }
        break;
    }
  }

  if (num_elements > 1 || num_doctypes > 1) {
    exception_state.ThrowDOMException(
        DOMExceptionCode::kHierarchyRequestError,
        String::Format("Only one %s on document allowed.",
                       num_elements > 1 ? "element" : "doctype"));
    return false;
  }

  return true;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.