Overstep the mark/line
Definition 1 of 2
Pro Tip 1/3Social Boundaries
This phrase is often used when someone violates unspoken social rules or etiquette. 
He definitely overstepped the mark by joking about her divorce.
Pro Tip 2/3Disapproval Implied
Using 'overstep the mark' implies that the speaker disapproves of the behavior. 
She overstepped the mark with her comments, and the audience was not pleased.
Pro Tip 3/3Cultural Sensitivity
What counts as overstepping the mark can vary greatly between different cultures. 
In their culture, arriving unannounced is seriously overstepping the mark.