The spandrel exists because other circumstances brought it into existence

Came across the term spandrel which I have seen used in the past. It is not something frequent enough to retain in working memory so I head to look it up. The definitions offered are all architecturally related:

A spandrel, less often spandril or splaundrel, is the space between two arches or between an arch and a rectangular enclosure.

This, and the last couple of times I have seen it referenced, however, in each instance they were referring to a spandrel metaphorically - a thing created as an entirely unintended outcome of other actions. In architectural terms, the goal is to create the strength achieved by use of an arch. The fact that using an arch creates a spandrel is entirely unintended.

It may be used for decorative purposes but the spandrel exists because other circumstances brought it into existence, not because it was inherently desired for itself. I wonder how often the inadvertent creation of an unitended spandrel, then leads, through Kauffman's adjacent possible, to the creation of something entirly new without original intention. An example of unconscious creativity.