How to make an Impossible level on Robot 64 (patched)
Basically make an impossible jump with an ice-cream there. then move arrow to where the ice-cream is and click "test here". Move around for a bit to save a checkpoint here then exit. Finally click "test at spawn" and spam e to get off the platform quickly. This will take to where the ice-cream is. You'll be able to publish the level without anyone able to beat it. You can also do this glitch to complete long levels you've created in seconds.
Quick Sellout moment go check out my level called "Pentagon Panic (very difficult)" for a challenge.
Edit: the level creator is closed for the time being so this might be patched when it gets released again.