Although every cactus I grow is the same, they are, of course, all different. It's the unusual that I particularly find interesting. Here is a page devoted to some of the unique characteristics and my names for them:

Crowning: This seems to occur after a trauma at the very top of the cactus, during a growth phase when the flesh is most tender. I believe that if strong sunshine scorches this developing area, then the top kind of implodes and forms a small hole. Like this:

Scorched hole

The result is that a new baby grows over the hole hence the name of "Crowning". Here's an example:

Turning Orange:  I think that this is another response, like the above, to being scorched with concentrated sunlight. The flesh turns orange at first, later it goes brown and it then hardens and becomes wooden. Here's an example:

Turning Orange

Other phenomenon will be explored further soon, these will include: The Leaf-Cutting Bee Visits, The Flower Hornet, Water-Towering