A very bad case of multiple packet losses would be the reason indeed. Shouldn't happen and it never happened to me with our current Anti-Lead - but 0 dmg due to very very very bad packet loss is not 100% unavoidable here (99.8%, 99.9% yes - but not 100% due to performance reasons). Example; Let's say it happens to 1 of 50 players (which is 2 of 100 -> 2%) and for those players it doesn't happen all the time, it only happens on say every 50th shotgun shot (which is 2 of 100 -> 2%); 2% of 2% is 0.04% which means it works 99.96% of the time.
When's the last time this happened to you though? We've constantly done small changes and adjustments (sometimes without notice) to further prevent cases like yours from happening.
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However, as explained in my previous post, there's no such thing as 'half damage' or 'quarter damage' in our Anti-Lead, because pellets aren't sent seperately. It's either full damage (for the total amount of pellets you hit) or none. Just to clear that up because that's what some people were thinking when they dealt like 10 or 20 damage. - It's not related to Anti-Lead. Just the randomness of the shotgun-spray.