The issue was that the player could slide off of one wall and "rebound" through another very slowly. You can see the behaviour in
It turns out the problem was that I used 'continue' in an inner loop when I meant to make it continue from the outer loop; this is the same issue as the AT&T Crash which paralyzed the United States' telecom infrastructure in the early 90s. So at least I'm in good company.
Anyway, I'm glad I fixed this bug without a ton of math required; Afterglow feels more solid now and I should be free to implement more functionality.
I will probably make another (higher quality) YouTube video eventually when I have more to show off.