I had the same thing happen to me several years ago. I had a nice doe lined up at about 18 yards. The arrow appeared to be headed right for the pump station. About five yards from the doe, it suddenly nosedived and skidded under her feet! As I walked over to retrieve my arrow I found a pencil sized limb with a cut in the bottom of it...sure didn't see that when I took the shot!
Then a couple years later I had the buck of a lifetime at 15 yards broadside. I came to anchor and the arrow was away. At about the halfway point the arrow suddenly veered UP! WTF! It went about four feet over his back! I was so focussed on the spot on the buck, I didn't see a little twig, maybe 1/8" in diameter between him and me. Just enough to deflect the arrow. He heard the arrow hit the brush behind him and ran UNDER me.
It happens if you hunt enough. I think we get so focussed on our spot that we just kind of miss those little obstructions between us and our targets