Revealing mistake: As Kai's squadron fly across camera, it cuts to Kai in his cockpit with his shadow on his right. But as the camera pans around to get a side view of him, his shadow moves in the other direction revealing them moving the light with the camera. (00:01:55)
Revealing mistake: In the hanger dock where Stanley is working, there are some purple lasers in the background going up and down like a security gate. But during their animation for a moment, they suddenly jump back to previous positions as the loop in the animation is not lined up correctly. (00:11:40)
Revealing mistake: When the prisoner on the line recognizes Stanley and starts shouting, his frame breaks loose and he falls down. It leans forward and falls to the ground splattering the prisoner all over the floor, but as it is falling, it hits against the frame in front of him which has Xev tied to it. It pushes the bottom of Xev's frame up making it swing and hit the frame in front of her, and enough that it would have crushed where her feet were. But there is nothing to stop the metal from touching the metal frame in front of it revealing that for that shot there was nobody on that frame. (00:22:55)
Revealing mistake: When it's Xev's turn at the Holo Court, the memory machine plugs into her head and starts playing her memories of her arranged wedding. The machine is showing video of her memories through her eyes. However in the video when she punches her would be husband in the face, the side and part of the back of her head comes into view on screen. Which doesn't make sense unless she's able to see her own back of her head. (00:29:10)
Revealing mistake: Stanley stops next to a window and contemplates if he should turn himself in and face a forced organ donation of 3, or face termination. In the close up shots, the green screening was poorly done as he has a red outline around his face and head. (00:33:05)
Revealing mistake: When Stanley doesn't make it back to the correctional facility in time and is locked out, it triggers his fugitive status for failing to turn himself in. Behind him and all through out the base, his image appears on screens along the walls like wanted posters. Only a little bit before as he is running down the hall trying to get back to the correctional facility in time, he passes by one of these screens and you can see this same image of him already on the screen but dim. Revealing that the production team already had the image set and ready to turn a back lighting on to reveal the image at the appropriate time. (00:34:00)
Revealing mistake: When the Cluster Lizards invade the awards ceremony, one of them grabs a boy and drags him across the floor for a moment before it cuts to the lizard slinging him around devouring him. In that shot, it is painfully obvious that the cluster lizard is slinging around a dummy, and the dummy doesn't even have a head. (00:39:25)
Revealing mistake: After the Cluster Lizard inside The Lexx eats Thodin's last man and The Key is transferred to Stanley, there is a shot from behind Xev and Stanley showing the lizard. The green screening is reflecting shades of green onto Xev's black restraining vest and part of Stanley's hat. (00:59:35)
Answer: From what I'm seeing, the answer seems simple. The robotic heads allow the control of the body they need. However, no mater how advanced you make a robot's body, it will never quite be as flexible and versatile in it's movements and actions as a flesh and blood human body can be in most cases. And if they do put the effort into creating a robotic body with the capabilities they need, and be durable enough to last... it would cost a lot of money. The Cluster has a large influx of constant prisoners that they use for feeding the Lexx and other needs, so human bodies are easy to come by and in vast quantities. Them simply producing robotic heads with the ability to interface with the human nervous system and placing them on the decapitated bodies of expendable prisoners is a hell of a lot more cost effective and easier to do with the technology they have. It's just a cheaper, more effective option for them to have thousands fo 790 Units in production and use without the cost of manufacturing a fully functional and durable robotic body for them.
Quantom X ★