Not precooking filling

All the ingredients (apart from the cheese) should be cooked before assembling and baking lasagna, whether it is made with meat or vegetables. In essence, you're just melting the cheese and baking the flavors together. Ground beef, hog, and ground poultry must all be precooked to 165 degrees Fahrenheit (via FDA). A foodborne sickness could result from forgetting to do this or missing it. Although it's uncommon, eating raw veggies might make you ill (via Eat This, Not That).

The major purpose of preheating vegetable fillings, however, is to combine the aromatics and dry out the veggies to reduce excessive, unwelcome moisture in the lasagna. If you want the veggies in your summer lasagna to retain some of their crunches, blanch them rather than sautéing them in oil. Vegetables can also be roasted, but to guarantee equal cooking, make sure they are all around the same size.