Grindstones treat curses of vanishing as enchantments, despite not getting rid of the curse. Replicate this by taking an item with only the curse of vanishing and no other enchantments and putting it through the grindstone, unlike a normal unenchanted item itll let you use the grindstone anyway, allowing an infinite loop of using the grindstone. This does not yield xp or benefit you in anyway however, but still seemingly unintended.