Those lag spikes in your screenshot are not caused by other players' pings. Other players pings, however, are impacted by the lag spikes. If the server is lagging and there are a lot of players with high pings, they have high pings because the server is lagging, not the other way around. If this is a frequent problem, perhaps you should look into getting the graffiti servers hosted in another location.
That being said, an individual player's lag is often visible to other players. His movement will be choppy and sometimes he will freeze midair. This does not cause lag for other players, but it can be annoying. This is more dependant on packet loss than ping. Perhaps, despite KnacK's "low" ping, he had severe packet loss and was lagging all over the map. Perhaps Sharon just made a mistake. In any case, unless this is repetitive behavior, I don't see why it's such a huge deal. It's nothing like the "good old days" when you'd get viewgibbed for capturing a flag.
