When Mitch is walking to work on his 40th birthday, he passes by the Ed Sullivan Theater, which prominently displays a sign for the Late Show with David Letterman. Since Mitch was born on September 8, 1952, that would make the date September 8, 1992. The Late Show didn't premiere until August 30, 1993.
After duke saves Mitch, Phil and Glen from the two cowboys they rented the equipment from duke cuts the ropes on their hands and Mitch says "Curly were so sorry we thought you were dead". However his mouth does not move.
At the very beginning of the movie, Mitch gets up at 5:16am, goes jogging with his "cow", then commutes by train from his home in New Rochelle to downtown Manhattan and arrives to work at 6:07am (see clock in background over his head). Can't be done.
In the shot of the train arriving at New Rochelle, the train pulls past the green 'New Rochelle' marker sign. However the train is heading in the opposite way of the writing on the sign, indicating that the train is in fact leaving New Rochelle, not arriving as implied.
When the map is on fire, the wind blows it around whilst the guys chase after it to put it out. However whilst they're running after it, the leaves on the trees and the bushes they run past don't move at all, proving that there is no actual wind.