Had someone mention adding tasks to stories in our sprint mid-sprint is messing up the sprint statistics... Can someone explain to me how one is supposed to know every task and approximately how long it will take to complete for a given story before even opening the code base up?

This is currently my major gripe with agile / scrum. How exactly you're supposed to instinctively know the solution to a complicated problem, as well as the steps to implement it, the approximate time it'll take, AND roadblocks you'll run into on DAY ONE? WHAT?

Too often does a 2 point story turn into a 5 point story because deep down it was a more complicated problem than originally thought, and a good scrum developer is supposed to... Either clairvoyantly known that or just allocate hours into unrelated tasks?

Someone help me out here