There are always going to be days when a previous professor keeps you late, or you wake up late, or it takes you too long to find a parking space, but the point here is do not be habitually late to class. Regularly arriving late to class signals a level of disrespect -- whether you mean to send that signal or not. If you have problems getting to class on time, find a way to solve them. And on those rare days when you do arrive late, remember to enter the room quietly and not make a big scene.