When Krankor's ship returns to Earth the morning after the first confrontation, a bird is perched on top of the spinning periscope. In the close-up, the bird flies away from the periscope.
The wires suspending the space ships are frequently visible.
Krankor is allowed to escape after he threatens to kill a boy. As he starts running up the stairs, the boy (who is standing next to him) starts following him up the stairs, realizes he's supposed to stay where he is, and stops.
The aliens' noses are obviously plastic; the edges are clearly visible in several close-ups.
When Krankor's ship shoots at people, some of the resulting explosions come from different directions; others never happen.
A very obvious zipper runs down the giant monster's back.
During an escape sequence at the end of the film, the British scientist's thin beard begins to peel off.
When the "guardian" monster's feet are on the video monitor, you can see the wood underneath the set.
Prince of Space claims the enemies' weapons have no effect on him, which they don't. Yet he still flinches and tries to avoid their fire when he is fired upon.
After the aliens make their presence known to the world, we find Wally and the kids on the street shining shoes. Wally is reading a newspaper. Jerry Tempura pulls up a few seconds later, the paper he is reading is gone and Jerry gives him one "hot off the press".
When Mickey defends boot-blacking, a woman shows up behind Wally and takes the telescope the kids were playing with off the set.
If the planet Krankor is 500,000 miles away from Earth, why hasn't it been noticed?
A gravestone in the cemetery that gets knocked over is clearly a hollow prop.