There appears to be a glitch where a Heavy shooting at a Scout, whether it's Sasha or Natascha, may miss the Scout for a short duration until bullets finally start hitting.
I, for one, can attest that in Badwater Basin, I was a Scout pushing the cart when a Heavy ambushed me. He point-blanked shot me for a good five seconds until I finally got hit and died instantly. And this happened in similar fashions a few times as well., sometimes allowing me to escape, sometimes not.
And no, there were no lag spikes, as everything else around us was moving quite smoothly.