the reason was....
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QUOTE
The sad part was that I was on the milan side, with all the brigate whatevers, well i was there and outside the san siro, at the gates,THEY DIDNT LET ME GO IN WITH MY BANNER, IT WAS TOO OFFENSIVE, **** U COPS, BLOODY SHITASS-
ANYWAYS, it was sad that they banned all ultras/milanistas banners and ours wouldve been the best, and all the fans protested against that by not singing, WHICH IMO SUCKED BADLY BUT I WAS SINGING WITH SOME OTHER GUYS, which made that we got into a fight with some dudes from brigate rossoner./fossa del leoni who wanted us to shut up but i told them that i didnt pay 250 euros to go and sit quiet at the derby and that my banners got banned as well so better take it and stop being babies, hell yeah, lucky for me they didnt understand most of what I said else I wouldnt be here. LOL
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from someone on another forum who went to the derby today