Seeing Red - S6-E19
Corrected entry: When Tara got shot, the hole in the window and Tara shows that the bullet has to have entered in a straight line, but Tara and Willow were on the second level and Warren was on the bottom level, and he shot upwards, not straight, so the bullet couldn't have entered in a straight line.
Correction: When a bullet travels though glass the trajectory will be changed. How much depends on many things like type of glass and the round fired. Since the round that hit Tara went though glass would be really difficult to say where it would hit after hitting the window.