I cannot understand why one of my folders has a text file icon superimposed. I haven’t seen this before.
I suspect the answer is reasonably obvious, and I will accept whatever humiliating sarcasm anyone wishes to deliver. 
I have attached a little screen shot for clarity.
Search for “icons” in the Help file (Help > Scrivener Help), and you will find the answer under “The Binder”. 
In short: it means that the folder has some text in it (i.e. when you toggle off the corkboard, there is some text behind it in the folder document itself).
Best,
Keith
Thank you!