Maybe...maybe not. Maybe you just imagined a coffin. Maybe it's a macabre Stan Lee cameo where he claws his way out of the grave wondering why people think he's dead. Could be anything really.
Actual answer:
The coffin in the commercial is for Peggy
Slightly less spoilery answer, but still technically a spoiler:
The person in THAT coffin doesn't die as a result of the Civil War.
Gentlemen, fellow nerds. Today is the day that I finally get to see Captain America: CIVIL WAR. Im so freaking excited. Today is going to be long because the showing isnt until 10:30. I hope I dont have a heart attack during the movie due to its sheer awesomeness.