Souls of the Departed - S5-E12
Corrected entry: In the graveyard scene while Emma is summoning Captain Hook, large orange cones billowing with fog smoke can be easily seen. The cones are strategically placed throughout the graveyard to produce the eerie fog sequences but are easily seen in many shots. (00:29:00)
Corrected entry: Hades states that Zeus is his older brother when actually he was the youngest among the first twelve gods. So Hades is older than Zeus. (00:27:55)
Correction: In Greek mythology, yes, but not in Once Upon a Time: The show freely adapts the source material, with numerous drastic changes to the original: The Queen of Hearts is the mother of The Evil Queen from "Snow White" and the Wicked Witch of the West, The Crocodile is the son of Peter Pan, Red Riding Hood and the wolf are one and the same, Peter Pan is a villain, etc.
Not only that, but it comes from the 1997 Disney film "Hercules" in which Zeus is the older brother. Which only compounds the mythology problem since Hercules is the son of Jupiter and Heracles is the son of Zeus. So Disney is just continuing its own mythology story.
Correction: This is not a mistake. The Underworld is full of such smoking cones, and they can be seen in several episodes. For example, in the first scene of "Labor of Love", the camera pans to a close-up of a cone, and then goes down the cone, to reveal the underground prison below. The cones seem to be some form of ventilation from the underground levels, where Hades dwells - the pit of hell, so to speak.