The paper with the "Iceman" headline that Mr. Freezy hands to Kuklinski has a story about Roy Demeo's body being found in the trunk of his car, though he is not yet dead in the movie and in fact Kuklinski and Mr. Freezy are discussing putting a hit on him in that scene.
When the Iceman is driving to make the hit on Marty Freeman, his gloves are on/off/on between shots.
When his brother calls it's a collect call from the Hudson County Jail but when he visits him a short time later his brother is being held in Trenton State Prison.
One hour, thirty four mins and thirty nine seconds into the movie when Richard Kuklinski is being arrested in his 1976 Lincoln continental with his wife, there is a December 2013 New Hampshire Vehicle inspection sticker that appears on the windshield when the camera zooms into their faces to see their reactions to the armed police officers approaching them.
When Richard meets Mr Freezy in the park, after he kills Leo, Mr Freezy gives him a newspaper with a headline that reads "THE ICEMAN MURDERS!!". There are two stories visible on the first page. One is about Roy Demeo's body having been found in the trunk of his car. If it wasn't for Mr Freezy commenting about Roy getting to them, we could've just assumed Richard had killed Roy off-screen. Also, the second story on the first page is titled "New mafia man" , but the text is about helicopters.
In the hospital scenes, "Cover Your Cough" posters are visible in the hallways even though that campaign did not start until the 2000's - many years after the film takes place.
In the hospital, there are visual fire alarms in the hallways. The visual fire alarms didn't appear in the buildings until 1990s under Americans with Disabilities Act of 1990.
Some of vehicles travelling on the street in 1976 were not produced until 1977 or later. The vehicles are as following Chevrolet Malibu (1978), Chevrolet Impala taxicab (1977), a parked Oldsmobile Delta 88 station wagon (1977). A double-parked Ford Econoline didn't have rectangular headlamps until 1979, following a minor facelift.
Early in the movie when Kulkinski is sitting in the back seat of Leo's "new" Buick Electra 225, the headliner and C-pillar lining is loose, ill fitted and wrinkled.