The following should be a valid curve, but returns nan for any progress:

    QEasingCurve curve(QEasingCurve::BezierSpline);
    curve.addCubicBezierSegment(QPointF(1.0/3.0, 0.0), QPointF(1.0-1.0/3.0, 1.0), QPointF(1.0, 1.0));
    qDebug() << test.valueForProgress(0.35);

The problem is that BezierEase::findTForX divides by factorT3, which is zero when the distance between the control points is 1/3 of the distance between the endpoints.

There are also likely issues in cases where the points are distanced such that factorT3 = factorT2 = factorT1 = 0, or the equation is quadratic and the discriminant is less than zero.