Yeah, I have been surprised at the slap back from reflected sound more than a few times.
Last winter, I shot some handguns with my brother in-law, who also let me fire his new 50-caliber "Desert Eagle" handgun. He lives on a farm, and we set up our range about 15 yards from an all-metal barn. I neglected to bring my hearing protection, and I could definitely tell that my left ear--the one closest to the barn--was significantly hurt by the reflected sound.