The Crypt

They didn't specifically mention two coffins, simply because it was plainly seen that there were two.
We didn't see them install one in the Old House. Nor did we see Willie drive back and forth to the Eagle Hill cemetery when Barnabas first took control of him, nor did we witness Willie tell him what year it was, who was living at Collinwood, etc., etc. But we know all of those things happened.
They didn't spell everything out. They didn't have to. The word "vampire" was never used until the 1795 episodes. Some things are plainly seen and don't require mentioning.

I never got the impression there were two coffins. I just figured there was some mistake made along the way, and the prop guys forgot where the coffin was supposed to be at a particular point in time.

As Scary Mary said, the coffins are identical, right down to the dent marks. I doubt Willie could replicate those from the original.

I'm surprised people are so insistent on there being two coffins. Dark Shadows was full of mistakes, everything from walls shaking, microphones coming into view, cameras being seen, stage hands coughing, laughing and chatting off-camera, and as it was pointed out by someone, the Josette portrait did move around and was in places it shouldn't have been...just like the coffin.

But yet, people are insisting this wasn't a mistake, as if just this once--no matter how incomprehensible--Barnabas somehow dug up a new coffin, when the old one was perfectly useable.

I've watched all the episodes on Sci-Fi when they ran, and I never once had the impression there were two coffins. It just makes since that after lying in a coffin for almost 200 years, Barnabas would take it with him to the Old House. That it magically reappeared in the mausoleum was just an error, that's all.

So your answer to all of my questions, which I have now posted twice, was that it was simply a continuity error? You are saying that the writers, directors and producers were so sloppy that they didn't think that viewers would wonder how the coffin, that was clearly in Barnabas's basement, was suddenly in the mausoleum while David was locked in there? Not only was the coffin in the mausoleum instead of Barnabas's basement while David was locked in there, but it was also in there when Sarah first showed David the secret room? If it was only one coffin, how did it go back and forth between the mausoleum and Barnabas's basement? This is not a simple gaff like seeing strings attached to a fake bat. This would be an insult to the intelligence of viewers. And your comment that you did not see a coffin in the basement on set is irrelevant. It is one hundred percent certain that Barnabas has a coffin in his basement. We have already seen that a dozen times. That is why Barnabas and Willie freaked out when the sheriff wanted to look into their basement. Surely you remember those scenes? You have not adequately answered these logical questions. Your only answer is that they were continuity mistakes. Which I do not buy. And you can talk yourself blue in the face but I think that most viewers understand that there were in fact two coffins within the context of thrstory, but the same prop used in both places.