When Don and Smith first hear the alien voice, the distance between them changes several times during he scene.
The colonists ship was the same ship used in many times previously. A blue filter was added to the film.
With everyone assembled and waiting for the spaceship to arrive at the gateway arch, Dr. Smith removes the explosive cherub from the side of the gateway. The wires are clearly still attached on the cherub's backside while now loosely extending from the gateway's side. Later, with Niolani still waiting at the gateway, Smith summons Will to help remount the cherub, but the wires are now neatly attached and hidden along the edge of the gateway's side.
Dr. Smith's extreme caution with handling the sculpture made of plastic explosive was unnecessary. Plastic explosives are notoriously stable until the proper detonation is applied. Compounds such as C-4 can be handled, molded, formed, dropped and even exposed to flame and set on fire without exploding. The value of plastic explosives lies in the very fact that it takes a deliberate act of detonation to set them off.
A ray gun in Major West's hand is destroyed on 2 separate occasions. On each occasion, the close-up of the gun exploding is the identical video footage from the same camera and angle, down to each spark and flame.