When Mare is walking to the crime scene in the woods, a policeman is seen wrapping crime scene tape that has already been started off screen and is wrapping it around the next tree. As she walks by the tape is now just being started on the first tree.
At the beginning of the episode, when she was getting out of her car "Mare" was walking normally without limping , then the next scene she started limping again.
After mass, when the priest and deacon are disrobing, they are both wearing black shirts. Permanent Deacons in the Archdiocese of Philadelphia, wear gray shirts with the clerical collars.
In the interrogation room, Brianna requests a lawyer, but Mare keeps trying to elicit information from her. Once a suspect requests a lawyer, police are legally prohibited from questioning or discussing the case with them until an attorney has arrived. Any evidence elicited after Brianna requested a lawyer would be inadmissible. An experienced detective like Mare would know this.
(at around 33 mins) When Mare leaves Siobhan's room, the sound of the closing door is clearly heard before the door actually shuts.