Using わかったもんじゃない The online dictionary Jisho says that this is a colloquial expression, but offers no examples of its use; and I can't find examples elsewhere online. I used it in a sentence intending to say, "We might stay at the club long enough to have breakfast with the band, but **who knows…** " I was aiming for a light-hearted joke since everyone present knew that we don't have the stamina to stay up that late. I was told that did not work in my sentence, that the expression is not light-hearted. Could someone please offer some guidance regarding when to use/not use ?

(or ) is a set phrase that always implies something bad will happen. For example, implies she would almost certainly do something unexpected and bad, although it is hard to imagine what she would do exactly or how awful it would be. In your case, did not work because you didn't know whether the result would be favorable or unfavorable. Simply, you could have said instead.