About 20 years ago, when I was working at a small publishing services company in Oakland, I typeset a book by Robert Sharer called The Ancient Maya, and all of the chapter numbers were in the Mayan script. I think they might even have been in the running heads, and therefore appearing on every page. In any case, over the course of working on this lengthy book (which had a lot of very picky layout needs and therefore took a lot of attention), I learned the numbering system up to 19.
I recognized the glyph immediately on the wall. I'm not sure if the person who drew the graffiti intended it as a Mayan number, but it was a reminder that all of us have strange, eclectic bits of knowledge we've gathered over the course of our lives. Who would have thought I would recognize a Mayan number glyph?