Mike Yaklich adds: The plane got off course during bad weather (zero visibility-- I think they were more or less trying to follow the Rhine, maybe the instruments were out). Anyway, when they made a forced landing in a field they found out that they were in Belgium, they tried to burn the documents but were interupted by the swift arrival of the local police, summoned by a farmer who saw them come down. The documents were partly burned but largely recoverable. Heads rolled on the German side when Hitler found out about this, particularly the commander of the officer in question (there had been strict orders not to put anyone carrying the plans in a position to be captured, and a flight from one German destination to another in the frontier zone, in a small plane (Fieseler Storch, I think) in the unpredictable weather of northern Europe in January (done basically just to save time in getting from point A to point B) was considered extremely poor judgement by the powers that be on the German side. Oddly enough, the punch line to the story is, of course, that this all turned out to be an enormous break for the Germans. The original Army plan for the invasion of France was basically a watered-down variant of the WWI Schliefen plan, which had as its objectives only to pry back the French/Allied armies from some of the Channel ports. Hitler himself had misgivings about it, but he considered that the French (the enemy whom many historians have subsequently written that he understood best) didn't have the stomach for a repetition of the 1914-18 bloodbath which had hit their country so hard, and would go to pieces at the first hard blow. In his mind the important thing was to attack and hit hard, the details were secondary (hence he kept pushing for such an attack at the earliest possible date, wanting to go in the fall of 1939 if possible, and then over the winter-- "We Remember" has been posting some of the details of these reschedulings and postponements). However, with the original plan originally compromised (the Germans weren't sure how much fell into enemy hands-- Belgium was after all neutral at this point, so the Germans became aware that they were at least partially burnt), the way was open for a new approach, which ultimately led to the brilliant "Sichelschnitt" maneuver of attacking through the Ardennes...
Greg Kelley adds: it was almost a comedy of errors in that the pilot/officer wasn't qualified for bad weather flying but wanted to impress his friend, and both officers turned out to be nonsmokers and hence neither had a match or lighter to actually burn the documents. They did get them smoldering by some means -- IIRC shoving them against hot engine parts -- and later after being captured one of the officers tried thrusting the papers into the stove at the police station they were taken to but was stopped after only signing some more page edges. As Mike said, the flight was entirely unnecessary and indeed reckless under the circumstances.