fuel management
It is a number one cause of accidents here in the US. I believe it is because most flight schools, and private flying lessons, never really teach fuel management. We always take-off with full tanks with just student and instructor aboard, fly for an hour or two, and land. We never worry much about how much fuel we have. Solo cross-countries are warned to make frequent fuel stops. So the student fies the planned trip with way more fuel than necessary, so running low on fuel never happens. Then he gets into the real world with longer trips, and a full boat, and...that's when it happens. Inadequate "real life" training.