I am aware of how floating point precision works in the regular cases, but I stumbled on an odd situation in my C# code.

Why aren't result1 and result2 the exact same floating point value here?

const float A;   // Arbitrary value
const float B;   // Arbitrary value

float result1 = (A*B)*dt;

float result2 = (A*B); 
result2 *= dt;

From this page I figured float arithmetic was left-associative and that this means values are evaluated and calculated in a left-to-right manner.

The full source code involves XNA's Quaternions. I don't think it's relevant what my constants are and what the VectorHelper.AddPitchRollYaw() does. The test passes just fine if I calculate the delta pitch/roll/yaw angles in the same manner, but as the code is below it does not pass:

X
  Expected: 0.275153548f
  But was:  0.275153786f

[TestFixture]
    internal class QuaternionPrecisionTest
    {
        [Test]
        public void Test()
        {
            JoystickInput input;
            input.Pitch = 0.312312432f;
            input.Roll = 0.512312432f;
            input.Yaw = 0.912312432f;
            const float dt = 0.017001f;

            float pitchRate = input.Pitch * PhysicsConstants.MaxPitchRate;
            float rollRate = input.Roll * PhysicsConstants.MaxRollRate;
            float yawRate = input.Yaw * PhysicsConstants.MaxYawRate;

            Quaternion orient1 = Quaternion.Identity;
            Quaternion orient2 = Quaternion.Identity;

            for (int i = 0; i < 10000; i++)
            {
                float deltaPitch = 
                      (input.Pitch * PhysicsConstants.MaxPitchRate) * dt;
                float deltaRoll = 
                      (input.Roll * PhysicsConstants.MaxRollRate) * dt;
                float deltaYaw = 
                      (input.Yaw * PhysicsConstants.MaxYawRate) * dt;

                // Add deltas of pitch, roll and yaw to the rotation matrix
                orient1 = VectorHelper.AddPitchRollYaw(
                                orient1, deltaPitch, deltaRoll, deltaYaw);

                deltaPitch = pitchRate * dt;
                deltaRoll = rollRate * dt;
                deltaYaw = yawRate * dt;
                orient2 = VectorHelper.AddPitchRollYaw(
                                orient2, deltaPitch, deltaRoll, deltaYaw);
            }

            Assert.AreEqual(orient1.X, orient2.X, "X");
            Assert.AreEqual(orient1.Y, orient2.Y, "Y");
            Assert.AreEqual(orient1.Z, orient2.Z, "Z");
            Assert.AreEqual(orient1.W, orient2.W, "W");
        }
    }

Granted, the error is small and only presents itself after a large number of iterations, but it has caused me some great headackes.