You can say, “It’s possible that this was an accident waiting to happen if the [furnace/boiler] was not properly maintained.”

This is a good example. “Their reservation” makes it sound like a guest’s reservation. Also, “staff” is singular (“don’t” should be “doesn’t”). I would say this: “If a hotel’s staff doesn’t check the reservation system every day, an overbooking is an accident waiting to happen.”