Big-Lipped Alligator Moment: The Danish version's musical scene with Dirch Mikkelsen (played by Dirch Passer) and a bunch of children.
Harsher in Hindsight: The film featuring a forest in the arctic would be considered merely illogical when it was released, but given climate change, that's becoming more and more a reality.
The quality of the footage used for Reptilicus is so much lower than the footage for the rest of the film that it looks like it was shot for a different movie, with no apparent effort to composite the shots to make it seem like the soldiers are even in the same universe as it, let alone shooting a thing right in front of them.
The green slime that is shot out of the monster's mouth is not only horribly out-of-place, but looks like stop animation when it happens.
The Danish version of the film has shots of Reptilicus flying that were cut out of the American version for looking too ridiculous. Think about that for a second.
Stock Footage Failure: Most notably the montage of ships allegedly destroyed by Reptilicus during its rampage. Mystery Science Theater 3000's riffing even points out that several shots were footage of ships sunk during the Suez Crisis, which would have been even more obvious to viewers watching in 1961.