There are times where you can stab a zombie even when looking at them and they seem to be aware you're there.
Sometimes this stabbing kills them and sometimes it does not.
Here's a clip from Joito:
It has happened the same to me before but I have actually killed the infected. (unfortunately I didn't record)
Not sure how to reproduce it, I have tried it doing it on purpose but no luck, it was always random and couldn't tell if there was anything happening at the same time.