While poking around adding products to my cart and then viewing the cart, I noticed that sometimes the trash icon didn't exhibit the usual hover action nor did it remove the product from the cart when clicked. I've determined that the cause of this is the back-to-top button. This button is visible only when one has scrolled down far enough. Until then the button is not visible due to opacity being set to 0. So even though the button is not visible, it is still "in front" of whatever is beneath it (blocking whatever action may be expected).

My "fix" was to change z-index to -1 for #back-to-top when the show class was not active (in stylesheet.css).