If it's every once in a while, I'm okay with it, especially if they'll give me the explanation that they had a bad day or whatever. But if it's a regular occurrence and they always have some excuse or the other, it gets a bit old. You get to the point where you want them to just admit that this isn't some rare event, them being mean to you, it's regular. It's not a behavior as much as it is who they are. Then, the issue can be addressed more properly.
“There are two novels that can change a bookish fourteen-year old’s life: The Lord of the Rings and Atlas Shrugged. One is a childish fantasy that often engenders a lifelong obsession with its unbelievable heroes, leading to an emotionally stunted, socially crippled adulthood, unable to deal with the real world. The other, of course, involves orcs.” ~ John Rogers