When the stunt crew was setting up a scene involving a character being thrown to his death from the third level of the mall, director Jim Wynorski volunteered to try the stunt himself as long as they set him up from the second level. He completed it successfully but found out he'd broken a rib in the process; Wynorski did not tell anyone he had gotten hurt and no one found about it during the remaining production time.
The film was allowed to shoot at a real California mall as long as they did not damage any facilities and had removed any traces of their presence before the mall opening time of 9AM. While the mall's head of security didn't like the filmmakers and was constantly accusing them of causing disrepair, the mall's owner was supportive of the film and made sure the production was able to complete its work on schedule.
The movie was originally theatrically released in March 1986 under its original title, "Killbots." It performed poorly during its initial release. The producers felt the movie's title might have disinterested audiences, who might think based on the original movie poster that it was a "Transformers"-like children's cartoon instead of a violent exploitation movie. After some time, the movie was re-released on video under its new title with over 15 minutes cut.
The book Gerrit Graham's character is reading is "They Came from Outer Space" which was edited by the film's director, Jim Wynorski. One of the stories in the collection is "The Racer" which was filmed as _Death Race 2000 (1975)_ featuring Mary Woronov.
The special effects crew actually built five remote controlled robots to serve as the Protector killbots. Three were required for the scenes of the robots together in the first half of the film, with two extras as backups in the event that the originals were damaged during any of the action sequences. In order to keep the robots looking realistic (as well as due to the film's budgetary constraints), they were constructed out of such items as wheelchair frames and pieces of conveyor belt. Excluding shooting laser beams, most of what the killbots are seen doing onscreen was the result of the effects crew operating them via remote control.
The budget for the film was very limited (around $800,000 total) but the director had no problems with this, as he was happy to work on a Roger Corman film and knew beforehand that Corman always kept expenses to a minimum.
The trivia item below may give away important plot points.
Despite the iconic VHS cover and ad campaign, the mechanical claw seen gripping the bloody shopping bag never appears in the film. Similarly, there is only one actual mutilation in the film (the exploding head scene). The rest of the victims die either by having their throats slit, being electrocuted, set on fire, or falling to their deaths.