Inferred Holocaust: Plan 9 is to stop humanity building a "solaranite bomb" and destroying the universe. Yet Plan 9 fails. What does this mean for the universe? Given that it's the Plan 9 Universe, total destruction may be merciful.
"You see? YOU SEE? Your stupid minds! Stupid! STUPID!"
Tor Johnson's damn blank-angry face when he becomes a zombie has achieved this status, even becoming a mask!
Older Than They Think: Criswell's opening line — "We are all interested in the future for that is where you and I are going to spend the rest of our lives" — is often held up as one of Wood's classically clunky lines. In actual fact, Criswell used that as the intro line for his TV show for several years before Plan 9 was filmed (and continued to do so afterward), though he usually gave it a much wryer delivery than he does in Plan 9 — witness it in action in his appearance on the Johnny Carson New Year's Special in 1966.
Though the cigar line is actually based on genuine UFO reports from pilots.
Bela Lugosi grows about a foot and hides his face behind a cape when his body double takes over, and we're not supposed to notice.
Woolseyism: The fact that some subtitled and dubbed versions have better writing than the actual movie is less of a show of the translators' skills and more damning evidence of the movie's own ineptitude. Just being able to truncate the redundant dialogue and make some phrases sound much more natural is an improvement already.