Many decades ago, when we were still high on Avalon Hill games, we played Air Assault on Crete.
That game has one side deploy its counters upside-down, including a number of dummy counters. The idea is that the opponent doesn’t know where units are deployed. The dummy counters were bright yellow ( see here: https://boardgamegeek.com/image/1254589/air-assault-creteinvasion-malta-1942 ). It turned out to be that when you looked at a certain angle at the board, you could see a yellow shine coming from underneath the counter.
So, my opponent was looking almost parallel to the board, to know where my dummy counters were placed. I complained it was not fair. He answered that the rules didn’t specify at what angle you had to look at the board, so it was fair game.
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