Yes you can overcook lemon bars. When overcooked, the lemon custard can curdle, so be careful not to whack up the heat higher than suggested or overcook them. While curdled lemon bars will probably taste OK, they don't look great. Overbaking these healthy lemon bars can also cause the tops to crack a bit! All ovens vary and I tested this recipe in 2 ovens and baking time varied from 15 to 20 minutes.