Continuity mistake: When Sylvia and Dr. Forrester are in the farm house, Sylvia cooks some eggs, then places the plates of eggs on the table. As she places them down you can see a plate with toast stacked on it. This plate disappears in the next shot of them both sitting down. (00:42:40)
Continuity mistake: The opening narration says the season is late spring, or otherwise a warm season of the year in Arizona, yet many people are wearing wool suits, coats and long-sleeved shirts during the day. At one point, the sheriff's dialogue even discloses that it is 92 degrees (and no air conditioning).
Continuity mistake: When the Beast first tears down the streets of New York, a lone police officer empties his service revolver at the beast before being eaten. A few minutes later, a group of a half dozen police officers pump their shotguns before firing at the beast - the officer who was already eaten after firing his revolver can be seen alive in the far left of the screen as one of the officers readying his shotgun.
Continuity mistake: When Duck Dodgers claims Planet X with his flag, you can see there is nothing next to him, but when Marvin the Martian comes along and makes his claim, an "X" tree has suddenly appeared.
Continuity mistake: Lieutenant Bowers questions the survivor of the first encounter with the alien, suspecting he might have just clobbered a love rival. The telephone rings; after it happens, the sheet in front of the suspect does a 180°. (00:09:35)