Here I'm using the undesigned coincidence concerning the green grass (Mark 6) and the time of Passover (John 6) for the feeding of the five thousand as an illustration of the weakness of theological invention theories.   In the course of it I "help out" the theological theorists by presenting three suggested symbolic allusions to Old Testament passages for the pillow on which Jesus was sleeping in Mark 4:38. This is intended to show that our ability to think up a clever-sounding connection between some apparently factual, casually narrated Gospel detail and an Old Testament passage is of no epistemic value. It's much, much too easy.   I'm taking the occasion from Dale Allison's recent discussion of this on Mythvision, where he said that it is "just as good" an explanation of the green grass in Mark that this is an allusion to Psalm 23 rather than evidence of eyewitness testimony.  

Here is the exchange on mythvision (cued up):  https://www.youtube.com/watch?v=ohvL5FoCgHA 

Here is an older article by Allison where he makes this same suggestion:  https://biblicalstudies.org.uk/pdf/irish-biblical-studies/05-3_132.pdf