…you can apply a preset to all your shots when you import them into Lightroom, such as, oh, let’s say your favourite monochrome treatment which you don’t use very often as you’re mostly a colour photographer. Lightroom then remembers this and has it selected as default on your next import so your lovely sunset shots are all turned into black and white. Which wouldn’t be so bad, except that the conversion to monochrome step isn’t a separate step in the develop history so you can’t undo it…
Fortunately, I still had the originals on the card and could re-import – without the monochrome step…