Yesterday Nintendo dropped the first major update for Super Smash Bros. Ultimate on Nintendo Switch. Version 2.0.0 brought numerous small tweaks, changes to fighters to make them more balanced, and of course, Piranha Plant as a free DLC fighter. Unfortunately, this update also appears to have brought some pretty major bugs along for the ride.
According to reports from fans, playing as Piranha Plant in All-Star Mode can potentially corrupt your save data, erasing your progress. Meanwhile, other users have reported problems with Duck Hunt and Mii Swordfighter. Apparently playing Century Smash as these two characters is also causing save data corruption. Hopefully this is a problem that Nintendo identifies and fixes soon, but for now, you should exercise caution and avoid these combinations of fighters and game modes.