If they warn peoples not to do that... It means...
Something terrible happened
Any A&E/ER/whatever place people go during emergencies will tell you about the things people stuff up their arses (a favourite I heard was expanding foam, it was really bad apparently and needed pretty major surgery as it adhered to things and burned).
This has the air of a trend or internet challenge. I am also not sure if there is something that makes this worse than other things -- I imagine uncooked potato could act as an irritant when it defrosts similar to handling them, and if they are unwashed (or the stupid American thing where they put dirt back on to make them).
Pictures thread but might as well do a video
Though generally the advice from doctors is if you insist on shoving something up your arse then make sure it has something to stop it going up any further (there is a functional reason a dildo might have balls on it) and seek medical attention if it goes wrong as trying to get it out yourself with tools and kitchen implements also has a nasty habit of instead grabbing hold of your lower intestines and other bits and at best cutting them and worst dragging them out and probably still with the thing stuck up there thus now requiring surgery rather than just a walk of shame.