Standing behind the line of fellow competitor's putt...

I never believed it to be a rule (though one it may have been)...but an etiquette not to be ignored. So if I ever absent-mindedly stand 'on-line' - when asked to move - I move. As I would expect any fellow player (companion, opponent or competitor) to do when asked regardless of whether it is allowed in the rules or not.
 
There was just some general rule chit-chat after our comp on Saturday - quite possible that the person who started the chat had seen the incident at the US-AM though.
Makes sense.
I saw the clip first then the thread!
It's an interesting discussion though, I had never seen this sort of behaviour in the flesh before, so hadn't considered how to handle it.
 
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