It happens especially in servers where players have a high ping. As you said, it doesn't happen often enough to get a demo (so it's not a big deal), and it's normal, so making a demo is not necessary. It happens to you and to me, and everyone.
It can happen for a combination of 2 reasons:
1. your ping is high -> client prediction
In the game there are 2 simulations, one in the client (what you see), one in the server (this is the 'real' game').
When you move, the simulation in the client makes you move at once (as if you played in lan), but in the server you haven't moved yet, till your command reaches the server, so you might believe you are behind the corner when you're not in reality.
2. your enemy's ping is high -> antilag
With antilag, you can aim at players where you see them, but a high pinger sees you in a position you don't have anymore, he sees you and shoots, then when the message reaches the server maybe you have moved behind the corner, but the server registers the hit, because he aimed at you when you were still visible for him.
In both cases you would still die even if all players played in lan, so it seems fair enough to me.
(Even if I remember years ago I had an argument vs a skilled etpro player about antilag, and he insisted in saying this is important, implying that I was too low to understand the impact in skilled vs skilled games)