Curve fitting is shaping a strategy's rules and settings to match the exact twists of a historical chart. It is closely tied to overfitting: the rules describe what already happened rather than a repeatable edge. A curve-fit strategy can show a near-perfect past and still have no predictive value.
Knowing the difference between a real pattern and a curve-fit one protects you from false confidence.
The app files could not be fetched. This is usually a network hiccup or an out-of-date cached copy of the site.