a (ticking) time bomb

A person, thing, or situation that can at any moment cause much havoc or result in a disastrous outcome. I'm telling you, this dirty money we're using to finance the campaign is a ticking time bomb! If anyone were to investigate how we got it, we'd all go to jail! Jenny's attracted to men who exude an air of danger, and her new boyfriend seems like a time bomb.

time bomb

A situation that threatens to have disastrous consequences at some future time, as in That departmental dispute is a time bomb just waiting to go off. This term alludes to an explosive device that is set to go off at a specific time. [First half of 1900s]