A Ticking time bomb:

A “ticking time bomb” basically means a situation that can happen at any time; something bad that will happen.

That man smokes too many cigarettes, drinks too much alcohol, eats too much junk food, and never exercises.  His doctor is worried about his heart and told him that he’s basically a ticking time bomb unless he changes his lifestyle.

Many experts say that the economy is like a ticking time bomb.  It’s OK now but it’s going to fail badly sometime in the near future.  We just don’t know exactly when.