I've had that happen to me twice at FedEx field in the past.
One year, I had a sign that said "I support Snyder and Spurrier" -- of course the joke being that I'm a Cowboys fan and I support the incompetent management of the team. Anyway, the sign was confiscated from me after about 10 minutes. In their defense, some Skins fans came to my defense when security took the sign away from me.
Another year, some guy in the parking lot was selling bootleg t-shirts, most of which said "F@&K DALLAS" and had the Calvin character peeing on the Star. But he also had a few "F@&K Commanders" shirts with Calvin peeing on the Skins logo. So I bought one and wore it into the stadium. Again, after about 5 minutes, security came up to me and demanded I take the shirt off. I pointed out two or three skins fans around me who had the anti-Dallas shirts on... one of which had a shirt that actually had the "F" word spelled out. Security said they didn't care about those, but I had to take off my shirt or I'd be kicked out of the stadium.