Some time ago I bought a Mammilaria elongata from Wilkos, and like all plants from there it was in pure peat. It looked healthy enough tho so I left it as is. But the other day I got myself some john innes n°2 and some grit/sand mix-like I used to use years ago,so I did some repotting. After doing the big opuntia (easier than it looked) I did the mamm.elongata. I unpotted it (rootball light as a feather) and a piece fell off. I presumed it was just an offset like I get with my mamm.vetula, but after I'd potted up the mother plant I had a look at the offset. It appeared to have a hollowed out callus that had long healed,and the attachment point on the mother plant was all dry and crusty. It must have suffered some rot at some point but survived!
Anyway I now have a potential new mamm.elongata start!
I'll keep an eye on it...