So I'm new to attempting really 11 exit runs and I like swimming underneath the stage on Donut Secret 1. Today, I got a random note block that killed me. Is this something that happens a lot? I've been swimming under this stage since the game came out and this never happened.

That glitchy block is always there, not sure exactly why but it will always be there. There are a few more like those around other levels, at the bottom of the screen like that note block or at the top of the screen stoping you from flying over certain levels.

That's really fun. Tons of play throughs of that level and never had it happen. Now my mission is to recreate it.

I experienced this many, MANY times, mashing faster won't do much until a certain point since there will be a row of those blocks you're bumping into as well (like you can create a row of 8-9 of those note blocks). I'm not sure if tit's because you're getting too low where you're very close to hit the death barrier already, no idea if people discovered why this is a thing, something I'd still need to find out... xD