The first time I saw someone use a bean bag was when I did a game with McColman. It looked cool and I thought about getting one, it's just not everyday that I end up at an official's supply store. It took me a while to get it
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The only downside to using the bean bag to mark POP on a punt is that if the recovery is long, you don't have ball boys like the CFL does to get your bean-bag and you end up running 70+ yards down the field for it.