Jack asks the NEST specialist if he got a look at the timer, and how much time is left on the bomb's countdown. He responds by saying 55 minutes, but the camera shows the timer counting down from 48:58.
The goof item below may give away important plot points.
A nuclear explosion only creates appreciable amounts of local fallout if the burst height is so low that the fireball touches the ground. In a sufficiently high air burst, the fission products are too widely dispersed to cause radiation poisoning. This is called 'global fallout' since the very small particles are quickly drawn up into the stratosphere and eventually dispersed worldwide. It is a lot less harmful since it is so diluted.
Taking the plane into a dive for a ground burst therefore would actually be the worst conceivable plan, generating heavy local fallout to contaminate any number of nearby communities, Edwards Air Force base and possibly parts of Los Angeles, depending on wind directions and precipitation. Even for a 40 kt bomb yield, the fireball radius would only be 270m (some 810 ft) though. In the 20 minutes of flight time, the Cessna 208 could have climbed to a height approaching its service ceiling of 25,000 feet, mostly avoiding any local fallout at all.