We already know that Nintendo is more than happy to remove uploaded Super Mario Maker 2 courses, usually for breaking the rules regarding glitching, abusive content, or sometimes for no clear reason at all, but it turns out that your levels might also be deleted when they haven't even been uploaded.
Kotaku reports that after creating a level containing the "black hole glitch" - which allows users to layer items on top of each other and endlessly clone them - it was soon deleted. The removal took place after saving the level (without uploading) and returning to the Coursebot menu; the following screenshot was presented.
Essentially, it appears that the game can detect glitches in your course and will instantly delete it. No matter how long you spent on the level, it would seem that it could be lost forever. Of course, if you're purposely trying to create glitches in the game, this would be pretty understandable, but users have reported the issue happening to them without them knowingly doing anything wrong.
In some cases, the game has allowed the user to re-enter the level and removing any offending parts. It seems that this is only the case if the error appears during the saving process, however, rather than after the course has fully saved.
If you're wanting to play around with glitches, you might want to be careful with any creations you're proud of; hopefully we won't see too many innocent players be affected.