I’m not sure I would call those bad examples bad triangulation; the question is rather are they USEFUL triangulations!

I never thought about that. I often use my late Aunt, which would be a close relationship. I need to change that!

Now that they can pull DNA from the air and water (Alaska Department of Fish and Game had an article about broadly pulling DNA from sea water recently and many media outlets carried the DNA from the air bit of late), how long until some scientist figures out that decaying bodies DNA is spread outside if the grave and can be sampled in the top layers of soil non-intrusively? That will open up so many opportunities for forensic genealogy!

Going round in circles in not triangulation!😹

Your example of the "good triangulation" to me doesn't make sense to me because whether you know the common ancestor or not should not matter. If what you are saying is right then good triangulation turns bad when you finally work out the relationships. This just doesn't make sense to me. I do agree that using close relatives, say siblings, is probably not so helpful, if you are going on the same definition of triangulation that you are using. I would like to throw into the mix a different kind of triangulation and that is when you and a match and shared match all match each other, but not necessarily on the same chromosomes (those using Ancestry and don't have these matches on say MyHeritage or GEDMatch, and that is probably most people will be in this situation). This to me is another type of triangulation. Now you have to be careful about using this information because, as you showed in a previous video, these may be false matches (or maybe rather shared matches) so maybe go down this rabbit hole a bit, but if you get nowhere, especially if we are talking about distant matches then they may not actually be matches or whatever matches at all, I think we you say said before due to recombination, how the SNPs are taken, and some other facts (I'll note have to go back and watch that other video). I hope that all makes sense. Maybe a video on using Ancestry's shared matches and triangulation and the limitations of it would be useful?