I checked the console, and I noticed that test methods for previous problems were getting recalled. I discovered it when running the Comparison with the Equality Operator test and saw a call to a cached version of the Shopping List test in the console, causing my answer to fail.
I did a hard page refresh to clear the cache, and the test then passed. I went back to previous exercises that I had failed, and in all but one case, my solution passed after the hard refresh.
I'm not sure if this is a bug in the test system or a byproduct of the browser (Chrome 55.0.2883.87 in my case), so I thought I would ask here before reporting it as a bug.