This question is a good one. If the question was "what would you do if you couldn't fail?" I'd say walk across an ocean. But if I knew I couldn't fail, that's different. Knowing something doesn't make it true. Because when you know something, you think you know it. Which means you might be wrong, but still think you know it. I know I won't fall off the top of a 10 storey car park, it doesn't mean I'm not scared to stand by the edge. So I'd always be wry that i knew something but it wasn't true, so I wouldn't do anything dangerous, like try to walk across an ocean.
My answer is to overcome my social anxiety. But that's because I know 100% that I will not and can not fail already, so I already have experienced the question.
Sorry for the long winded answer