The History Channel has an interesting program called "History verses Hollywood." Each episode examines a popular movie in light of historical accuracy. There were several inaccuracies in "The Longest Day", especially the ending where the troops storm up a hill through a breach in the wire. Didn't happen.

Some of the other glitches included American paratroopers returning fire as they were machine gunned in their chutes. Didn't happen because of the way our paratroopers equipment was rigged. Surviving Rangers didn't like how their attack up the cliff was portrayed as a failure. It wasn't - they found and destroyed the guns shortly after reaching the cliff tops.

This isn't meant to demean the movie, just to point out that like almost every other movie based on historical events, Hollywood tends to change the story to fit their own purposes or because they just don't know. The book is one of my favorite military histories and the movie is good, just not completely accurate.