Author Existence Failure: A famous example. Bela Lugosi died after filming only a few seconds of footage completely independent of the plot, and was rather obviously replaced by Wood's wife's chiropractor, who covered everything below the eyes.
Beam Me Up, Scotty!: Eros never said "A particle of sunlight contains many atoms!"; the actual quote is "A ray of sunlight is made up of many atoms!".
Common Knowledge: The flying saucers are actual toy flying saucers rather than the pie tins and paper plates that they're often mocked as.
Science Marches On: What appears to be another of the film's science blunders is Eros stating that after the H-bomb, the only explosion left to be discovered is the Solaranite, which has a pretty major omission in that no-one in this universe seems to have conceived of an antimatter bomb. It's hard to entirely fault Wood for not adding this in, however, as almost nobody outside of the scientific community had heard of antimatter in the mid-50s.
Strange Minds Think Alike: This films has a lot of similarities with Invisible Invaders. In both films the aliens are scared about the humans experimenting with weapons of mass destruction and technology they don't truly understand, so they reanimate corpses in order to kill the living until humanity surrender. Both films were released in the same year and although 'Plan 9...' was filmed 3 years earlier, 'Invisible Invaders' appeared on theaters a month before. At first 'Invaders' got much better reviews and became a TVCult Classic, but the film felt into obscurity while 'Plan 9' became a huge cult sensation in the 80's.