For heaven's sake... how in the world would you know it's the millionth bit that's corrupted without scanning all 999,999 other bits at least once?!?! Do you d

For heaven's sake... how in the world would you know it's the millionth bit that's corrupted without scanning all 999,999 other bits at least once?!?! Do you data streams always come magically tagged with sentences like "Hey, watch out for bits 999,998-1,000,000, but all my other bits are intact so you can skip them!" or something??! You somehow "know" the error is in the final 3 bits and not in the earlier 999,997 bits because... magic??