The night before the Challenger launch in January 1986, Morton Thiokol engineers told NASA the O-rings on the solid rocket boosters had never been tested below 53°F and were likely to fail at the forecast 36°F. Management at Thiokol overruled them under pressure to launch. The orbiter broke up 73 seconds after liftoff. Seven crew died.
The principle: When a schedule conflict reaches an engineer who has never flagged a concern before, the schedule is wrong, not the engineer. The cost of a delay is finite and visible; the cost of a failure is open-ended and obscured by everyone's hope that the engineers are being paranoid.