a wild card
1. someone or something that you do not know much about and whose behaviour in the future you cannot be certain of The real wild card is the undecided vote, which accounts for 18 to 25 percent of the electorate. The company is fast gaining a reputation as the wild card of Wall Street because of violent fluctuations in its profits.
2. if someone gets a wild card or is a wild card in a sports competition, they are allowed to enter the competition without passing the usual tests She was included in the European team as a wild card.