When you fling the cube to hit the button on the wall there's a chance it'll hit it twice, making the first new cube that drops get fizzled and replaced. This isn't a big deal and doesn't impact the puzzle at all, but if anyone wants to prevent this I found that you can by putting a small deactivation delay (such as with the delay tool in BEEmod or the Hammer equivalent) on the button. Three seconds or whatever the minimum is should do it.

I really like this collection so far, Ossy. Very clean, fun small puzzles. Thanks for contributing!