It's annoying that you can't click as soon as you reach the checkpoint. You become able to go again after some arbitrary delay and there's nothing that tells you when you can click, so if you want to go as soon as possible you have to just keep clicking really fast to see when you go.
Also, I found an exploitable bug. You can just keep holding Z throughout the whole game because you can have negative stars. You don't need any actual stars to buy the shield. I have -383 stars right now.
Edit: I also want to point out that almost all the medals have the wrong icons