Dear Reader:
Fix in the works for Undertale's nasty save-erasing glitch
Quick notice for Switch Undertale owners: there is a bug that can lead to accidental loss of save data if you don't select a user when you startup the game. A fix has been submitted, but for now, please make sure you select a user! (Don't press B to back out of this screen!) pic.twitter.com/ldTxbdf3Xq
— UNDERTALE (@UnderTale) September 18, 2018
Some Undertale players on Switch have noticed an unfortunately disastrous bug that will delete your save file. If you go to fire up the game and back out of the user select screen, your save data will be deleted. Don't select the game at all unless you plan on picking a profile and playing. If you accidentally go to this screen and don't want to play the game, pick a profile anyway, let the game load up, and then exit out.
While a fix for the issue has been submitted, the above method of protecting your saves is the only option as of right now.