Can someone elaborate on the idea changing after being accepted? Do you go a month of working on your idea, only to realise you're at a dead end - and be asked

Can someone elaborate on the idea changing after being accepted? Do you go a month of working on your idea, only to realise you're at a dead end - and be asked to think of something else? Do you fall back to one of your second ideas? Do mentors suggest working on something else? I'm curious - cheers.