I left-clicked only once to advance the (more) prompt. However, this input simultaneously did two actions.
Specifically, my missiles (rocks) had run out, so I was faced with the (more) prompt. I left-clicked *once*. In one fell swoop, the (more} prompt was dismissed, the list (to choose which missile to reload with) blipped on my screen, and a brown potion was unintentionally loaded into my sling. It was quite alarming, especially since a hostile harpy was watching.