There's a difference between a lapse in memory and being knocked unconscious. It's not at all uncommon to simply not remember the moment of impact and a second or so after, whatever it is that's doing the impacting.

Eh, this was a pickup game - no helmets, just kids playing. I was actually walking away from the box because I had to leave so I'm not really sure why it was thrown at me. Didn't find out either, just scowled and left. And it was only a few seconds of being unaware. I wasn't completely limp, just was walking out and then I was suddenly on my hands and knees.