Why does YAW cause Roll as well?

Hi People,

I am trying to yaw a ship without rolling.

Code (CSharp):

1.  
2. private void GetMouseInput () {
3.         lockZ = true;
4.             // roll = Input.GetAxis ("Roll") * a.speedRoll * Time.deltaTime;
5.         pitch = Input.GetAxis ("Vertical") * a.speedYaw * Time.deltaTime;
6.         yaw = Input.GetAxis ("Horizontal") * a.speedPitch * Time.deltaTime;
7.     }
8.  
9. private void UpdateRotation () {
10.         yaw = Mathf.Clamp (yaw, -0.5f, 0.5f);
11.         pitch = Mathf.Clamp (pitch, -0.5f, 0.5f);
12.         roll = Mathf.Clamp (roll, -0.5f, 0.5f);
13.         t.Rotate(pitch, yaw, roll);
14. }

When pitching all is fine, the roll is untouched. But as soon as i YAW (eg move left or right) the Z-rotation (roll) gets adjusted as well. I have no idea why this is and it's driving me slightly nuts at this point. Any help is very welcome.