Hit Bethany over the head with a shovel unintentionally thinking she was Alison.
Buried Bethany thinking she was Alison and that Spencer had hit her.
The night Bethany escaped Radley and wandered into the DiLaurentis' backyard, Mona had followed her thinking she was Alison. Mona was very angry and finally, thinking about everything Alison did to her, she picked up the shovel and hit Bethany from behind. Mona left the scene and was surprised later to find Alison ragged and scared but alive.
Melissa revealed to Spencer why she was protecting her. She explains in a video that she saw Spencer walk back to the barn dragging a shovel behind her. Melissa sees a blonde girl in a yellow top but not her face and believes her to be Alison's dead body. Desperate to save Spencer she rolls Bethany into the shallow grave Alison was in and buries her not knowing she was alive. The only person she told about what she did was Spencer, and her father, Peter.