started making character with his pose, now mirror doesn't work properly So I made a character without using mirror and placed him in a certain pose. Now I want to sculpt it with a mirror, but the mirror itself is mirroring something else. 
On the screenshot you can see that the single blue dot should be on the other side of the head, but it's not. That's because at the start I deformed the head.
Reset your scale and rotation with hotkey Ctrl + A  before you start sculpting.