When Slim returns the page from Mitch's mistress, Darcelle, we see a close-up of Slim entering the number on the cell phone. The cell phone is different than the cell phone she was using in the previous and subsequent shots.
Slim's weight fluctuates from scene to scene. This is especially noticeable when Slim & Gracie first arrive at their rental house. Slim is rather thin. In the next scene, when Gracie comments that she is not that slim, she is noticeably heavier.
When Slim dials 33 the first time, she gets Darcelle, the next time she gets her husband. However, the first time she dials 33, she does it on Mitch's phone. When she dials 33 to get Mitch, it is on her phone.
When Slim goes in to kill Michael she puts on big rings and bandages over her hands. Creating knuckle-dusters. When the police arrives she has taken them all off. Michaels wounds will not be consistent with her story, proving that she used a "weapon" in the fight and that it was premeditated. She will most likely be convicted for murder.
Throughout their relationship, Mitch has, without hesitation or fear, repeatedly assaulted Slim with and without weapons. During the final confrontation, Mitch enters his apartment and learns Slim is inside. Although he still thinks he has access to a pistol, he picks up the phone and dials 911.