Spot the failure.

Cross-check every instrument

The airplane is in a descending right turn. The magnetic compass shows the turn, and the GPS below it confirms it. As the view pans across the panel, the attitude indicator and turn coordinator both show the right turn, and the altimeter and VSI show the descent. One instrument doesn't agree with the rest: the heading indicator doesn't move through the turn.

Why it isn't the vacuum pump

A vacuum pump failure takes the attitude indicator and heading indicator down together. Here the attitude indicator is working fine through the turn, so the vacuum system is doing its job and only the heading indicator has failed.

Why resetting it won't help

Resetting the heading indicator to the compass fixes precession, where the heading indicator slowly drifts off but still turns with the airplane. This one doesn't turn at all, so resetting it won't help.

The compass and the heading indicator

The compass reads the earth's magnetic field and is the airplane's true direction source. The heading indicator is a spinning gyro. Once it's set to the compass, its rigidity in space lets it hold a steady, easy-to-read heading, but precession makes it drift, so it needs periodic realignment. In this video it has failed completely and no longer moves at all.

The inclinometer

The inclinometer shows coordination, not descent. It's the one instrument with nothing to cross-check it against, but it does move slightly in the video. If you ever doubt it in flight, a little deliberate rudder should push the ball off center.