An accident waiting to happen.

'An accident waiting to happen' is how you describe a situation that is dangerous and could cause an accident. Everyone knows it's a dangerous situation, but usually no one does anything to fix it until an accident has already happened.

Example:
-They really need to get a lock on this pool gate. It's an accident waiting to happen!
-You're so right! I'll go tell the pool managers.

Fender bender

A 'fender bender' is a minor car accident where someone bumps into another car and causes a small amount of damage. Now that fenders, now called bumpers, are painted the same color as the rest of the car, instead of being black like they used to be, even a little fender bender can be quite expensive to fix.

Example:
-Yesterday I had a little fender bender!
-Did you do any damage to the other guy's car?
-No, just to mine!