When Kolchak storms out of the office he forgets the photograph of the dog on the desk. In the very next scene he has it in his hands.
Kolchak alleges that the crown jewels of France were stolen by the Diablero. In fact, the French crown jewels were stolen by rioters in 1792, but almost all were later recovered, and are now on display at the Louvre. Only the Sancy Diamond and the French Blue Diamond were lost, although it is now believed that the latter was re-cut, and is now known as the Hope Diamond.
Although Dr. Temple spells the creature (singular)'s name (Diableros) when she first meets Carl, everyone refers to it as "Diablero" without the "S", even Dr. Temple when she meets with Carl a second time.
Kolchak's voiceover states that the story happens in mid-November, but the weather is bright and sunny. Chicago, on average, sees snow by late October.
When Kolchak takes a photograph of the 'dead' dog it moves an ear.
It's supposed to be early winter in Chicago (mid-November) yet no one is dressed for the weather.
Kolchak takes numerous pictures of the Diablero but we never see him develop these photos and we are never told that Native American sorcerers are incapable of being photographed.