I am grateful that customer contacted me instead of giving up! It made me aware of the problem so I could fix it and I was able to refer her to my other online shops so she could procure the pattern she wanted without having to wait.
I could not figure out why that happened, although I was able to reproduce the error when I tested the checkout process. I was still recovering from a nasty cold at the time and so I deactivated that listing until I felt well enough to fix it (the others I tested seemed to be functioning properly).
I reactivated that listing and tested it again today (along with some others just to be sure). It seems to be functioning properly today, as do the others I tested. I am assuming it was just a glitch in the system.
I will keep an eye on it myself, but I also greatly appreciate being contacted when someone encounters a problem on my site.