Ha, this isn't something I expected to come up during my Super Mario Odyssey travels, but here we are. As found by Twitter user DefinitivNichtSascha, apparently there's a way for Cappy to actually kill Mario.
In the video, they capture a Piranha Plant enemy, then proceed to spin Cappy and squish said plant. After doing said deed Mario seems to be dead and buried, but the game continues with Cappy swishing about in the air, looking for a master to reside on that no longer exists.
It's easily countered by fast traveling so it's not game-breaking -- no, we can chalk this one up in the humorous column. This game is so deep that there are myriad ways people are trying to break it!