I am watching this episode and I noticed what looks like something on the camera lens; I say that because the artifacts move with the camera in the bottom left. I was hoping someone here could confirm it’s officially a part of the series or if it’s just my video from where ever I sourced it.
Edit: it also appears again at 27:53.
I only have the PAL DVD set to refer to (so I think the scene in “By Inferno’s Light” you’re referring to occurs around 15:50) and I cannot see this artefact.
Looks like it might have been added by whatever re-encoding was done between the NTSC DVDs and your copy.