mistake #1 - forget to use `weak` memory modifier for the delegate variable

hard-skills-mistake-1.swift

protocol UserDetailsViewControllerDelegate: class {
  func update(with user: User)
}

final class UserDetailsViewController: UIViewController {
  var delegate: UserDetailsViewControllerDelegate?
  // some code below ...
  @IBAction func didTapContinueButton(_ sender: UIButton) {
    let user = // get new instance of the User or use the existing one
    delegate?.update(with: user)
  }
}