Slippage is the difference between the price you expected and the price your order actually filled at. It happens because prices move between the moment you send an order and the moment it executes, and because a large order may need to walk through several price levels. Slippage can be negative or positive, but in fast or thin markets it is more often a cost.
The app files could not be fetched. This is usually a network hiccup or an out-of-date cached copy of the site.