In Denver, you can't have mags above 20 rounds. It's a completely useless city council "ordinance" (not even a real law IMO), but per the wording of the statute you can get up to 6 months in jail if you are in violation. It's also incredibly ambiguously written - I had a buddy of mine who is a lawyer for the Department of Homeland Security take a look at it a couple years ago, and he felt that it was so poorly written that under some interpretations it might even make pistols like the Glock illegal, as they have 20+ round magazines "available."

Edit -- fortunately I don't live in Denver anymore, and can have all the 30+ round magazines I want.