Chamberlain has assured me that Level 5 of Thumper becomes unfair, and stupid, and not-fun.
So I beat the Level 4 boss (super-hard!), and move on to Level 5, my hackles raised, my defenses up, as the game introduces a new mechanic. And I keep on truckin' and havin' a fine time.
I beat the mid-boss, kept on thumping my way through, and after beating Level 5, Stage 19, Stage 20 began and this happened.
I had never seen this track before, never taken these precise turns before, and I ran it perfectly on my first try. Those are twenty very, very sweet seconds of gameplay.