The river on the wrong wall

An original fictional scene in an invented projection room.

When the projector started, the river ran across the ceiling. White ripples passed over the lamps and broke against a ventilation grille. The operator swore softly and reached for a switch. From the back of the room, a child asked him not to stop.

He left the picture running while he adjusted the mount. The river travelled down the wall, lifting a row of coat hooks into its current, and finally settled on the screen. Everything was now where it should have been. The banks stood upright; the water moved between them; the title could be read without turning one's head.

The child looked disappointed. I understood why, although the corrected picture was much clearer. For a few seconds, the room itself had seemed to be underwater. Now the river had returned to being a view we could sit apart from and watch.

How does the writer contrast the accidental projection with the corrected picture?