That depends on the section of the test. Critical reading you can get several wrong, writing depends on your essay score, and math you typically can't get any wrong.
I made 1 mistake and math and ended up with 790. I was quite annoyed with myself when I walked out knowing exactly which question I messed up too.
Did it hurt your future prospects?
Ruined my life.
Just kidding. It would be impossible to tell what, if any, effect it had. Do I feel like it actually had any effect? Not really.