system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] The Flash is sitting down, calmly eating dinner. I draw a bead on the back of his head with a sniper rifle and fire a supersonic bullet. What happens?

The bullet is going to hit the Flash before its sound can reach him, and neither I nor the bullet are in his visual field. So the first point at which he can possibly be alerted to the danger is when it first touches the back of his head. What happens next and why?

Follow-up question: If the Flash easily survives unharmed, how exactly do DC speedsters *ever* get hurt? Or fail at all?
gpt
This has happened several times to both Barry and Wally across various mediums. They feel the pressure of the bullet, and move out of the way.

As to how they can be hurt? User error. The Speed Force isn't a power that's always on. They can and do make mistakes, and Wally in particular is known to be more headstrong than he is actually smart (though he's not an idiot).

Hell, one of the first arcs of the New 52 has Barry literally overthink all the different ways his current scenario could go while a bullet is coming at him, to the point where he fails to completely move out of the way and gets grazed by it.