There have been times when the jet jockey simply forgot to lower them. But seeing the wheels in landing position still doesn't mean they'll stay that way when the plane is set down. The landing gear could fold halfway down the strip, or a wheel just go bouncing off on its own, reason enough for keeping the ship in sight until it heads for the taxi line. Any foul up, and the crews move out immediately. Crashes caused by wheel trouble usually aren't too serious.
Pilots are stunned and shaky but able to walk away with an assist from the rescuers.
There is always the threat of fire, however. Brake failures could pile up a plane in the soft earth beyond the runway. All tactical aircraft in the Fleet Marine Force are equipped with a hook for snagging the arresting cable on a carrier. When a pilot landing at El Toro discovers his brakes are gone, he merely drops the hook before crossing an arresting wire, the weight of the plane snaps a shear pin on each side of the runway and the plane has 24 tons of anchor chain by the tail. It stops em.