The first time I was introduced to the $\Box$ symbol in my first year of university I immediately thought it's a neat way how to mark that I have read and understood the proof:
As I later learned, to my surprise, that's not at all the origin or the purpose of the symbol. To this day, I use it that way - reading a text without ticking the tombstones would just feel wrong!
Of course, other people found this usage funny, some even thought I can't possibly be serious about it, which seems surprising, after all, it is a checkbox! This leads to the following question:
Question: How common is this? Or rather, have you encountered this usage at all, or am I commiting mathematical blasphemy and causing Paul Halmos to turn over in his grave?
This is, I suppose, a question that is possibly unsuitable for this site, in which case I apologize, just always wondered about this and don't know a better place to ask than Math.SE.