"We were surprised to find a significant effect of Python experience on the probability of making a very specific error (base = 0.13, OR = 1.44, p < 0.5). Instead of [8, 9, 0] for their last line of output, 22% of participants wrote [8] (for both versions). After the experiment, one participant reported they mistakenly performed the “common” operation on lists x btwn and y btwn instead of x and y because it seemed like the next logical step"

I made this mistake too. In fact, even after reading that most people wrote [8] instead of [8,9,0], it took me a solid minute or so to figure out how anyone could possibly think the correct answer was [8,9,0].

This is something I've noticed happens very often while debugging. I make certain assumptions about the code and even when staring it right in the face it's hard to see where my assumptions differ from the code as written.

My assumption in this case was "why would you calculate the between lists if you weren't going to use them right away".