Speaking of gosubs and gosub_scenes, I spent all day yesterday tracking down a couple of randomtest failures.
It wasn’t until the wee witching hours, this morning that I finally stumbled upon the answer:
I also discovered on my own that this maximum of two loop backs even applies when the origins is a different scene.
So, thank you @Jacic, for the assist, three years after you made your thread! 
I really wish I knew this before building up and fleshing out my codex.