I was at the arcade with some friends when two white guys came in and started shouting at the person playing ITG2. For example, they would shout whatever step judgement they saw (e.g. "Fantastic! Excellent! Excellent! Great!" etc.) I think a lot of people who play dancing games are used to this, but I decided today I was going to do something about it.
So I walked up to the one doing the shouting and I said "What are you doing?" He didn't hear me the first time, saying "huh?" so I repeated "What are you doing?" I don't remember his exact words, but he said something along the lines of "I'm just checking out this machine. I've never seen it before. This guy is pretty awesome isn't he?" Then I said, with raised eyebrow "Yeah... but I think shouting at him like that distracts him." So he said "Oh really? Okay..." and from there on we started having a conversation. He rhetorically asked if the game gave a good workout, whether I was better than the guy playing now, what kind of music do I like best, whether I was gonna play too, etc.
This experience sort of surprised me, because I had assumed that people who do the kind of stuff they did had malicious intent. Instead, these two seemed well intentioned, though perhaps a bit drunk.