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)
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.