Check your check-in cut off times before you leave your house

For most airlines, the cut off time to check-in for an international flight is an hour before departure.  This means if you are within that hour, it is at the discretion of the airline whether you get checked in or not.  If you are late, be very nice to the agent and do not argue as that will get you on the next flight, which unfortunately for you maybe tomorrow.

In Europe, they are much stricter than in the U.S. with this check-in rule.  Also, in Europe, depending on the country to which you are traveling, the time may be less than an hour.  A look at your airline Web site or a phone call will clarify this information.