A couple of days ago, the UK Supreme Court ruled that Boris Johnson's advice to the Queen, which led to the prorogation of Parliament for 5 weeks ahead of the October 31st Brexit date, was unlawful. Therefore, the prorogation has been cancelled and Parliament has been recalled.
However, my understanding (which may be incorrect) is that the act of proroguing Parliament is a royal prerogative power of the monarch. Although, they are supposed to take guidance on the matter from their Government.
So, does that mean, at least in theory, that the Queen could overrule the Supreme Court's decision to cancel the prorogation, by citing her royal prerogative. Essentially: "Yes, the advice given to me may have been unlawful, but I still want Parliament prorogued anyway."