UKC

I just fixed a puncture on my wife’s mtb. It was a weird double bulge on the inside of the inner tube. I didn’t take a photo of it but it looked exactly like the photo in this Reddit thread: https://www.reddit.com/r/bikewrench/comments/i1e7cw/my_tubes_always_punctur...

I replaced the the tube and the tyre as well, as the old one was quite worn. Couldn’t see any problem with the rim tape or spokes. The puncture was at the same part of the tube as above, at the join. There doesn’t seem to be any consensus on the thread above. Had the bike for years without this ever happening before. We picked the puncture up on a long, fast, rocky descent yesterday. Luckily it must have happened right at the end of the ride so I only noticed this morning. 
Any ideas what’s causing this? I really want to avoid being eaten alive by midges whilst fixing a puncture. 

Pinch flat? They usually result in 2 holes and fast rocky descents are prime pinch flat terrain.

They are on the inside face of the tube - i.e. facing the rim. Would that be a pinch flat? Also, they are one behind the other rather than side-by-side. That is to say, if a snake had done it he’d have been at right angles to the tube and on the rim-facing surface. 

If it was a pinch flat then maybe I should run them at a higher pressure. They have been at about 40 psi, which seems high but the sidewall says 30 - 55 (Schwalbe Black Jacks 26 x 2.1). 

Higher pressures or go tubeless!

Looks and sounds like rim tape issue (pushing too far through tape into end of spoke) but if rim tape looks OK its a weird one, is it cloth or plastic rim tape?

I took the tyre off again to have a good look. It’s plastic tape. I can see the indentations in the tape where it goes over the spoke holes but it’s only slight. What did strike me as odd though is that the rim tape looks different in the area where the puncture was, as though the tube hadn’t been in contact with the rim there, so maybe the tube had some inconsistency which allowed it to bulge.