hostage to fortune means an act, commitment, or remark which is regarded as unwise because it invites trouble or could prove difficult to live up to. In other words, you can say: Don't make unwise comments or promises that you can't keep.

'Hostages to fortune' comes from a famous quote from the writer and philosopher Francis Bacon 'He that hath wife and children hath given hostages to fortune, for they are impediments to great enterprises, either of virtue or mischief.' Here it means that once you have a wife (or husband) and children or other people depending on you, you can't go off on a reckless, great adventure. Today we also use it to describe an action or statement that is risky because it could cause you trouble later. 'The Prime Minister was extremely cautious, saying nothing inflammatory and giving no hostages to fortune.' 'He was very cautious with his words and gave no hostages to fortune.'