The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock. Harry released Neville, though he was unaware of doing so. He was jumping down the steps again, pulling out his wand, as Dumbledore, too, turned towards the dais.
It seemed to take Sirius an age to fall: his body curved in a graceful arc as he sank backwards through the ragged veil hanging from the arch.
Harry saw the look of mingled fear and surprise on his godfather’s wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind, then fell back into place.
Harry Potter and the Order of the Phoenix by JK Rowling, chapter 35 "Beyond the Veil"
Sirius had a look of "fear and surprise" as he fell through the veil. Did he know what the Veil was and what it did? Or did the fear come from being hit at all?