Cholla intergrades

If you find a cholla that just looks weird compared to its neighbors chances are it's an intergrade - either a two-parent hybrid or some level of back-crossing to one parent or the other*. One helpful aid to finding the relationship is to ID the taxon of the chollas that are found nearby. When encountering the odd ones make a note, and if possible separate observations, of the cholla taxa in the area.
There are numerous records of cholla intermediates in herbarium collections; many of those are noted by Donald Pinkava in this Canotia article.
*Or to a third taxon - theoretically possible